Semiconductor Device and Electronic Device

ABSTRACT

The circuit includes a first transistor; a second transistor whose first terminal is connected to a gate of the first transistor for setting the potential of the gate of the first transistor to a level at which the first transistor is turned on; a third transistor for setting the potential of a gate of the second transistor to a level at which the second transistor is turned on and bringing the gate of the second transistor into a floating state; and a fourth transistor for setting the potential of the gate of the second transistor to a level at which the second transistor is turned off. With such a configuration, a potential difference between the gate and a source of the second transistor can be kept at a level higher than the threshold voltage of the second transistor, so that operation speed can be improved.

This application is a continuation of copending U.S. application Ser.No. 15/632,704, filed on Jun. 26, 2017 which is a continuation of U.S.application Ser. No. 14/804,772, filed on Jul. 21, 2015 (abandoned)which are all incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice, a display module, and an electronic appliance.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, a manufacture,or a composition of matter. Another embodiment of the present inventionrelates to a semiconductor device, a display device, a light-emittingdevice, a power storage device, a memory device, or a driving method ormanufacturing method thereof.

2. Description of the Related Art

A circuit which can be applied to a driver circuit of a memory device,an image sensor, a display device, or the like has been developed. Inparticular, a circuit formed using transistors having the same polarityhas been actively developed. A technique relating to such a circuit isdisclosed in Patent Document 1.

In Patent Document 1, a potential difference between a gate and a sourceof a transistor is decreased gradually. When the potential differencebetween the gate and the source of the transistor is equal to or lowerthan the threshold voltage of the transistor, the transistor is turnedoff, and a node in a circuit is brought into a floating state.

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2005-050502 SUMMARY OF THE INVENTION

In a conventional circuit, a potential difference between a gate and asource of a transistor is decreased gradually, and thus the draincurrent of the transistor is also decreased gradually. Therefore, thetime required for change in a potential of a node in a circuit is long,and thus high-speed operation is difficult. Furthermore, W/L of thetransistor needs to be increased, which makes it difficult to reduce alayout area. In addition, it is difficult to shorten the rise time andthe fall time of a signal.

An object of one embodiment of the present invention is to provide anovel semiconductor device. Another object of one embodiment of thepresent invention is to provide high-speed operation or to provide aconfiguration which enables it. Another object of one embodiment of thepresent invention is to reduce a layout area or to provide aconfiguration which enables it. Another object of one embodiment of thepresent invention is to reduce a driving voltage or to provide aconfiguration which enables it. Another object of one embodiment of thepresent invention is to shorten the rise time and the fall time of asignal or to provide a configuration which enables it.

One embodiment of the present invention does not necessarily achieve allthe objects listed above and only needs to achieve at least one of theobjects. The description of the above objects does not disturb theexistence of other objects. Other objects will be apparent from and canbe derived from the description of the specification, the drawings, theclaims, and the like.

One embodiment of the present invention is a semiconductor deviceincluding a first transistor, a second transistor, a third transistor,and a fourth transistor. One of a source and a drain of the firsttransistor is electrically connected to a first wiring, the other of thesource and the drain of the first transistor is electrically connectedto a second wiring, one of a source and a drain of the second transistoris electrically connected to a third wiring, the other of the source andthe drain of the second transistor is electrically connected to a gateof the first transistor, one of a source and a drain of the thirdtransistor is electrically connected to a fourth wiring, the other ofthe source and the drain of the third transistor is electricallyconnected to a gate of the second transistor, one of a source and adrain of the fourth transistor is electrically connected to a fifthwiring, and the other of the source and the drain of the fourthtransistor is electrically connected to the gate of the secondtransistor.

One embodiment of the present invention is a semiconductor deviceincluding a first transistor, a second transistor, a third transistor,and a fourth transistor. One of a source and a drain of the firsttransistor is electrically connected to a first wiring, the other of thesource and the drain of the first transistor is electrically connectedto a second wiring, one of a source and a drain of the second transistoris electrically connected to a third wiring, the other of the source andthe drain of the second transistor is electrically connected to a gateof the first transistor, one of a source and a drain of the thirdtransistor is electrically connected to the third wiring, the other ofthe source and the drain of the third transistor is electricallyconnected to a gate of the second transistor, one of a source and adrain of the fourth transistor is electrically connected to a fourthwiring, and the other of the source and the drain of the fourthtransistor is electrically connected to the gate of the secondtransistor.

One embodiment of the present invention is a semiconductor deviceincluding a first transistor, a second transistor, a third transistor,and a fourth transistor. One of a source and a drain of the firsttransistor is electrically connected to a first wiring, the other of thesource and the drain of the first transistor is electrically connectedto a second wiring, one of a source and a drain of the second transistoris electrically connected to a third wiring, the other of the source andthe drain of the second transistor is electrically connected to a gateof the first transistor, one of a source and a drain of the thirdtransistor is electrically connected to a fourth wiring, the other ofthe source and the drain of the third transistor is electricallyconnected to a gate of the second transistor, one of a source and adrain of the fourth transistor is electrically connected to the thirdwiring or the fourth wiring, and the other of the source and the drainof the fourth transistor is electrically connected to the gate of thesecond transistor.

Note that in one embodiment of the present invention, a gate of thefourth transistor may be connected to the first wiring or the secondwiring.

One embodiment of the present invention is a display module includingthe above semiconductor device and an FPC.

Another embodiment of the present invention is an electronic applianceincluding the above display module, and, an antenna, an operationbutton, or a speaker.

One embodiment of the present invention can provide a novelsemiconductor device. One embodiment of the present invention canprovide high-speed operation or a configuration which enables it. Oneembodiment of the present invention can reduce a layout area or providea configuration which enables it. One embodiment of the presentinvention can reduce a driving voltage or provide a configuration whichenables it. One embodiment of the present invention can shorten the risetime and the fall time of a signal or provide a configuration whichenables it.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a semiconductor device.

FIGS. 2A and 2B each illustrate an example of a semiconductor device;

FIGS. 3A and 3B each illustrate one example of a semiconductor device.

FIGS. 4A and 4B each illustrate an example of a semiconductor device.

FIGS. 5A and 5B each illustrate an example of a semiconductor device.

FIGS. 6A and 6B each illustrates an example of a semiconductor device.

FIG. 7 is a timing chart showing an example of operation of asemiconductor device.

FIGS. 8A and 8B illustrate an example of a semiconductor device.

FIGS. 9A and 9B illustrate an example of a semiconductor device;

FIGS. 10A and 10B each illustrate an example of a semiconductor device.

FIGS. 11A and 11B each illustrate examples of a semiconductor device.

FIGS. 12A and 12B each illustrate an example of a semiconductor device.

FIGS. 13A and 13B each illustrate an example of a semiconductor device.

FIGS. 14A and 14B each illustrate an example of a semiconductor device.

FIG. 15 illustrates an example of a semiconductor device.

FIG. 16 is a timing chart showing an example of operation of asemiconductor device.

FIG. 17 illustrates an example of a structure of a semiconductor device.

FIG. 18 illustrates an example of a semiconductor device.

FIG. 19 illustrates an example of a display device.

FIG. 20 illustrates an example of a semiconductor device.

FIG. 21 illustrates an example of a semiconductor device.

FIG. 22 illustrates an example of a semiconductor device.

FIG. 23 illustrates an example of a semiconductor device.

FIG. 24 illustrates an example of a display module.

FIGS. 25A to 25G illustrate examples of an electronic appliance.

FIG. 26 illustrates an example of a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be describedbelow in detail with reference to the accompanying drawings. Note thatthe present invention is not limited to the following description and itis easily understood by those skilled in the art that the mode anddetails can be variously changed without departing from the scope andspirit of the present invention. Therefore, the present invention shouldnot be interpreted as being limited to the description of embodimentsbelow.

One embodiment of the present invention includes, in its category,devices such as an imaging device, a radio frequency (RF) tag, a displaydevice, and an integrated circuit. The display device includes, in itscategory, a display device including an integrated circuit, such as aliquid crystal display device, a light-emitting device in which alight-emitting element typified by an organic light-emitting element isprovided in each pixel, an electronic paper, a digital micromirrordevice (DMD), a plasma display panel (PDP), and a field emission display(FED).

In describing structures of the present invention with reference to thedrawings, the same reference numerals are used in common for the sameportions in different drawings in some cases.

Note that in this specification and the like, part of a diagram or atext described in one embodiment can be taken out to constitute oneembodiment of the invention. Thus, in the case where a diagram or a textrelated to a certain portion is described, the context taken out frompart of the diagram or the text is also disclosed as one embodiment ofthe invention, and one embodiment of the invention can be constituted.The embodiment of the present invention is clear. Therefore, forexample, in a diagram or text in which one or more active elements(e.g., transistors), wirings, passive elements (e.g., capacitors),conductive layers, insulating layers, semiconductor layers, components,devices, operating methods, manufacturing methods, or the like aredescribed, part of the diagram or the text is taken out, and oneembodiment of the invention can be constituted. For example, from acircuit diagram in which N circuit elements (e.g., transistors orcapacitors; N is an integer) are provided, it is possible to constituteone embodiment of the invention by taking out M circuit elements (e.g.,transistors or capacitors; M is an integer, where M<N). For anotherexample, it is possible to take out some given elements from a sentence“A includes B, C, D, E, or F” and constitute one embodiment of theinvention, for example, “A includes B and E”, “A includes E and F”, “Aincludes C, E, and F”, or “A includes B, C, D, and E”.

Note that in the case where at least one specific example is describedin a diagram or text described in one embodiment in this specificationand the like, it will be readily appreciated by those skilled in the artthat a broader concept of the specific example can be derived.Therefore, in the diagram or the text described in one embodiment, inthe case where at least one specific example is described, a broaderconcept of the specific example is disclosed as one embodiment of theinvention, and one embodiment of the invention can be constituted. Theembodiment of the present invention is clear.

Note that in this specification and the like, a content described in atleast a diagram (or may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, when a certain content is described in adiagram, the content is disclosed as one embodiment of the inventioneven when the content is not described with a text, and one embodimentof the invention can be constituted. In a similar manner, part of adiagram, which is taken out from the diagram, is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. The embodiment of the present invention is clear.

In addition, contents that are not specified in any text or drawing inthe specification can be excluded from one embodiment of the invention.Alternatively, when the range of a value that is defined by the maximumand minimum values is described, part of the range is appropriatelynarrowed or part of the range is removed, whereby one embodiment of theinvention excluding part of the range can be constituted. In thismanner, it is possible to specify the technical scope of one embodimentof the present invention so that a conventional technology is excluded,for example.

In this specification and the like, it might be possible for thoseskilled in the art to constitute one embodiment of the invention evenwhen portions to which all the terminals of an active element (e.g., atransistor), a passive element (e.g., a capacitor), or the like areconnected are not specified. In other words, one embodiment of theinvention can be clear even when connection portions are not specified.Further, in the case where a connection portion is disclosed in thisspecification and the like, it can be determined that one embodiment ofthe invention in which a connection portion is not specified isdisclosed in this specification and the like, in some cases. Inparticular, in the case where the number of portions to which theterminal is connected might be plural, it is not necessary to specifythe portions to which the terminal is connected. Thus, it might bepossible to constitute one embodiment of the invention by specifyingonly portions to which some of terminals of an active element (e.g., atransistor), a passive element (e.g., a capacitor), or the like areconnected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the presentinvention can be clear. Further, it can be determined that oneembodiment of the present invention whose function is specified isdisclosed in this specification and the like. Therefore, when aconnection portion of a circuit is specified, the circuit is disclosedas one embodiment of the invention even when a function is notspecified, and one embodiment of the invention can be constituted.Alternatively, when a function of a circuit is specified, the circuit isdisclosed as one embodiment of the invention even when a connectionportion is not specified, and one embodiment of the invention can beconstituted.

In this specification and the like, when it is explicitly described thatX and Y are connected, the case where X and Y are electricallyconnected, the case where X and Y are functionally connected, and thecase where X and Y are directly connected are included therein.Accordingly, without being limited to a predetermined connectionrelation, for example, a connection relation shown in drawings or text,another connection relation is included in the drawings or the text.

Here, X and Y each denote an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, or alayer).

Examples of the case where X and Y are directly connected include thecase where an element that enables an electrical connection between Xand Y (e.g., a switch, a transistor, a capacitor, an inductor, aresistor, a diode, a display element, a light-emitting element, or aload) is not connected between X and Y, and the case where X and Y areconnected without the element that allows the electrical connectionbetween X and Y provided therebetween.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. A switch is controlled to be on or off. Thatis, a switch is conducting or not conducting (is turned on or off) todetermine whether current flows therethrough or not. Alternatively, theswitch has a function of selecting and changing a current path. Notethat the case where X and Y are electrically connected includes the casewhere X and Y are directly connected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; amemory circuit; and/or a control circuit) can be connected between X andY. Note that for example, in the case where a signal output from X istransmitted to Y even when another circuit is interposed between X andY, X and Y are functionally connected. Note that the case where X and Yare functionally connected includes the case where X and Y are directlyconnected and the case where X and Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or another circuit provided therebetween), X and Y arefunctionally connected (i.e., the case where X and Y are functionallyconnected with another circuit provided therebetween), and X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween). That is, inthis specification and the like, the explicit description “X and Y areelectrically connected” is the same as the description “X and Y areconnected”.

Note that, for example, the case where a source (or a first terminal orthe like) of a transistor is electrically connected to X through (or notthrough) Z1 and a drain (or a second terminal or the like) of thetransistor is electrically connected to Y through (or not through) Z2,or the case where a source (or a first terminal or the like) of atransistor is directly connected to one part of Z1 and another part ofZ1 is directly connected to X while a drain (or a second terminal or thelike) of the transistor is directly connected to one part of Z2 andanother part of Z2 is directly connected to Y, can be expressed by usingany of the following expressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit configuration is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Other examples of the expressions include, “a source (or a firstterminal or the like) of a transistor is electrically connected to Xthrough at least a first connection path, the first connection path doesnot include a second connection path, the second connection path is apath between the source (or the first terminal or the like) of thetransistor and a drain (or a second terminal or the like) of thetransistor, Z1 is on the first connection path, the drain (or the secondterminal or the like) of the transistor is electrically connected to Ythrough at least a third connection path, the third connection path doesnot include the second connection path, and Z2 is on the thirdconnection path” and “a source (or a first terminal or the like) of atransistor is electrically connected to X at least with a firstconnection path through Z1, the first connection path does not include asecond connection path, the second connection path includes a connectionpath through which the transistor is provided, a drain (or a secondterminal or the like) of the transistor is electrically connected to Yat least with a third connection path through Z2, and the thirdconnection path does not include the second connection path” Stillanother example of the expression is “a source (or a first terminal orthe like) of a transistor is electrically connected to X through atleast Z1 on a first electrical path, the first electrical path does notinclude a second electrical path, the second electrical path is anelectrical path from the source (or the first terminal or the like) ofthe transistor to a drain (or a second terminal or the like) of thetransistor, the drain (or the second terminal or the like) of thetransistor is electrically connected to Y through at least Z2 on a thirdelectrical path, the third electrical path does not include a fourthelectrical path, and the fourth electrical path is an electrical pathfrom the drain (or the second terminal or the like) of the transistor tothe source (or the first terminal or the like) of the transistor”. Whenthe connection path in a circuit structure is defined by an expressionsimilar to the above examples, a source (or a first terminal or thelike) and a drain (or a second terminal or the like) of a transistor canbe distinguished from each other to specify the technical scope.

Note that these expressions are examples and there is no limitation onthe expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, and a layer).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention will be described.

A structure of a semiconductor device of one embodiment of the presentinvention will be described with reference to FIG. 1. Note that oneembodiment of the present invention is not limited to the structuredescribed below.

A semiconductor device illustrated in FIG. 1 includes a circuit 100. Thecircuit 100 has a function of controlling the potential of a wiring 112based on the potentials of a wiring 111, a wiring 113, a wiring 114, anda wiring 115. The circuit 100 outputs a signal based on the potentialsof the wiring 111, the wiring 113, the wiring 114, and the wiring 115 tothe wiring 112. The potential of the wiring 112 is controlled by thesignal. As described above, the circuit 100 serves as a logic circuit ora sequential circuit.

The circuit 100 includes a transistor 101, a transistor 102, atransistor 103, a transistor 104, a capacitor 105, and a capacitor 106.A first terminal (also referred to as one of a source and a drain) ofthe transistor 101 is connected to the wiring 111, and a second terminal(also referred to as the other of the source and the drain) of thetransistor 101 is connected to the wiring 112. A first terminal and asecond terminal of the transistor 102 are connected to the wiring 113and a gate of the transistor 101, respectively. A first terminal, asecond terminal, and a gate of the transistor 103 are connected to thewiring 114, a gate of the transistor 102, and the wiring 114,respectively. A first terminal and a second terminal of the transistor104 are connected to the wiring 115 and the gate of the transistor 102,respectively. A first terminal and a second terminal of the capacitor105 are connected to the wiring 112 and the gate of the transistor 101,respectively. A first terminal and a second terminal of the capacitor106 are connected to the gate of the transistor 101 and the gate of thetransistor 102, respectively.

The semiconductor device in this embodiment has a connection relationdescribed above, and thus can have a novel configuration.

Note that the gate of the transistor 101, the second terminal of thetransistor 102, the second terminal of the capacitor 105, or a firstterminal of the capacitor 106 is denoted by a node ND1. In addition, thegate of the transistor 102, a second terminal of the transistor 103, thesecond terminal of the transistor 104, or the second terminal of thecapacitor 106 is denoted by a node ND2.

Note that the potentials of the wirings 111, 113, 114, and 115 arecontrolled in such a manner that a signal, voltage, or the like is inputto the wirings. The potentials of the wirings 111, 113, and 114 eachhave a high level and a low level for convenience. That is, signals eachhaving a high level and a low level are input to each of the wirings111, 113, and 114. A high-level potential is VH, and a low-levelpotential is VL (VH>VL). The potential of the wiring 115 is VL. Notethat the potential of the wiring 115 may have a high level and a lowlevel. That is, a signal having a high level and a low level may beinput to the wiring 115.

Note that the wirings 111, 113, and 114 may each be referred to as aninput terminal. The wiring 112 may be referred to as an output terminal.The wirings 111, 112, 113, and 114 may each be referred to as a signalline. The wiring 115 may be referred to as a power supply line.

Transistors which can be used as the transistors 101, 102, 103, and 104are described. Note that one embodiment of the present invention is notlimited to the transistors described below.

As the transistors 101, 102, 103, and 104, a transistor includingamorphous silicon in a channel formation region, a transistor includingpolycrystalline silicon in a channel formation region, a transistorincluding single crystal silicon in a channel formation region, atransistor including an oxide semiconductor in a channel formationregion, a transistor including a compound semiconductor in a channelformation region, and the like can be used. In particular, a transistorincluding an oxide semiconductor in a channel formation region (alsoreferred to as an OS transistor) has a higher mobility and an extremelylower off-state current than a transistor including amorphous silicon ina channel formation region. Accordingly, the channel width of thetransistor can be made small, leading to a reduction in a layout area.

The conductivity type or the polarity of the transistors 101, 102, 103,and 104 is described. Note that one embodiment of the present inventionis not limited to the conductivity type or the polarity described below.

The transistors 101, 102, 103, and 104 preferably have the sameconductivity type. Alternatively, all the transistors included in thecircuit 100 preferably have the same conductivity type. Furtheralternatively, all the transistors provided over the same substrate asthe circuit 100 preferably have the same conductivity type. Thus,simplification of a manufacturing process, improvement in yield,reduction in manufacturing cost, or the like can be achieved.

It is particularly preferable that the transistors 101, 102, 103, and104 be n-channel transistors. Alternatively, it is preferable that allthe transistors included in the circuit 100 be n-channel transistors.Further alternatively, it is preferable that all the transistorsprovided over the same substrate as the circuit 100 be n-channeltransistors. This enables a transistor including an oxide semiconductorin a channel formation region (also referred to as an OS transistor) tobe used. In FIG. 1, the case where the transistors 101, 102, 103, and104 are n-channel transistors is illustrated. However, the transistors101, 102, 103, and 104 may be p-channel transistors. Alternatively, allthe transistors included in the circuit 100 may be p-channeltransistors. Further alternatively, all the transistors provided overthe same substrate as the circuit 100 may be p-channel transistors. FIG.26 illustrates a structure in which the transistor 101, the transistor102, the transistor 103, and the transistor 104 in FIG. 1 are replacedwith a transistor 101 p, a transistor 102 p, a transistor 103 p, and atransistor 104 p, respectively. The transistors 101 p, 102 p, 103 p, and104 p are p-channel transistors. Furthermore, also in a configurationother than the configuration in FIG. 1, an n-channel transistor may bereplaced with a p-channel transistor as in the configuration in FIG. 26.

Note that description is made on the assumption that the transistors101, 102, and 103 are n-channel transistors for convenience.

Functions of the transistors 101, 102, 103, and 104, and the capacitors105 and 106 are described. Note that one embodiment of the presentinvention is not limited to the functions described below.

The transistor 101 controls conduction and non-conduction between thewiring 111 and the wiring 112. When the wiring 111 and the wiring 112are brought into electrical contact, the potential of the wiring 111 issupplied to the wiring 112, and the potential of the wiring 112 iscontrolled based on the potential of the wiring 111. When the potentialof the wiring 111 is at a high level, the potential of the wiring 112 isincreased. In particular, when the potential of the node ND1 is higherthan the sum of the potential of the wiring 111 at a high level and thethreshold voltage of the transistor 101, the potential of the wiring 112is increased to VH. When the potential of the wiring 111 is at a lowlevel, the potential of the wiring 112 is decreased to VL.

The transistor 102 controls conduction and non-conduction between thewiring 113 and the node ND1. When the wiring 113 and the node ND1 arebrought into electrical contact, the potential of the wiring 113 issupplied to the node ND1, and the potential of the node ND1 iscontrolled based on the potential of the wiring 113. When the potentialof the wiring 113 is at a high level, the potential of the node ND1 isincreased. In particular, when the potential of the node ND2 is higherthan the sum of the potential of the wiring 113 at a high level and thethreshold voltage of the transistor 102, the potential of the node ND1is increased to VH. Thus, the potential of the node ND1 is set to alevel at which the transistor 101 is turned on. When the potential ofthe wiring 113 is at a low level, the potential of the node ND1 isdecreased to VL. Consequently, the potential of the node ND1 is set to alevel at which the transistor 101 is turned off.

The transistor 103 controls conduction and non-conduction between thewiring 114 and the node ND2. When the wiring 114 and the node ND2 arebrought into electrical contact, the potential of the wiring 114 issupplied to the node ND2, and the potential of the node ND2 iscontrolled based on the potential of the wiring 114. When the potentialof the wiring 114 is at a high level, the potential of the node ND2 isincreased. However, since the gate of the transistor 103 is connected tothe wiring 114, when the potential of the node ND2 is increased to avalue obtained by subtracting the threshold voltage of the transistor103 from the potential of the wiring 114 at a high level, the transistor103 is turned off. Thus, the node ND2 is brought into a floating state.In this manner, the potential of the node ND2 is set to a level at whichthe transistor 102 is turned on, and the node ND2 is brought into afloating state. Furthermore, when the potential of the wiring 114 is ata low level, the transistor 103 is turned off, leading to non-conductionbetween the wiring 114 and the node ND2.

Note that as illustrated in FIG. 2A, a first terminal and the gate ofthe transistor 103 may be connected to a wiring 116 and the wiring 114,respectively. The potential of the wiring 116 is preferably VH. Notethat the potential of the wiring 116 can have a high level and a lowlevel. In FIG. 2A, the transistor 103 controls conduction andnon-conduction between the wiring 116 and the node ND2. When the wiring116 and the node ND2 are brought into electrical contact, the potentialof the wiring 116 is supplied to the node ND2, and the potential of thenode ND2 is controlled based on the potential of the wiring 116. Whenthe potential of the wiring 116 is VH or at a high level, the potentialof the node ND2 is increased. However, since the gate of the transistor103 is connected to the wiring 114, when the potential of the node ND2is increased to a value obtained by subtracting the threshold voltage ofthe transistor 103 from the potential of the wiring 114 at a high level,the transistor 103 is turned off. Thus, the node ND2 is brought into afloating state. In this manner, the potential of the node ND2 is set toa level at which the transistor 102 is turned on, and the node ND2 isbrought into a floating state.

Note that as illustrated in FIG. 2B, the first terminal and the gate ofthe transistor 103 may be connected to the wiring 114 and the wiring116, respectively. In FIG. 2B, the transistor 103 controls conductionand non-conduction between the wiring 114 and the node ND2. When thewiring 114 and the node ND2 are brought into electrical contact, thepotential of the wiring 114 is supplied to the node ND2, and thepotential of the node ND2 is controlled based on the potential of thewiring 114. When the potential of the wiring 114 is at a high level, thepotential of the node ND2 is increased. However, since the gate of thetransistor 103 is connected to the wiring 116, when the potential of thenode ND2 is increased to a value obtained by subtracting the thresholdvoltage of the transistor 103 from the potential of the wiring 116, thetransistor 103 is turned off. Thus, the node ND2 is brought into afloating state. In this manner, the potential of the node ND2 is set toa level at which the transistor 102 is turned on, and the node ND2 isbrought into a floating state. When the potential of the wiring 114 isat a low level, the potential of the node ND2 is decreased to VL. Thus,the potential of the node ND2 is set to a level at which the transistor102 is turned off.

Note that as illustrated in FIG. 3A, the first terminal and the gate ofthe transistor 103 may be connected to the wiring 113. In FIG. 3A, thetransistor 103 controls conduction and non-conduction between the wiring113 and the node ND2. When the wiring 113 and the node ND2 are broughtinto electrical contact, the potential of the wiring 113 is supplied tothe node ND2, and the potential of the node ND2 is controlled based onthe potential of the wiring 113. When the potential of the wiring 113 isat a high level, the potential of the node ND2 is increased. However,since the gate of the transistor 103 is connected to the wiring 113,when the potential of the node ND2 is increased to a value obtained bysubtracting the threshold voltage of the transistor 103 from thepotential of the wiring 113 at a high level, the transistor 103 isturned off. Thus, the node ND2 is brought into a floating state. In thismanner, the potential of the node ND2 is set to a level at which thetransistor 102 is turned on, and the node ND2 is brought into a floatingstate. Furthermore, when the potential of the wiring 113 is at a lowlevel, the transistor 104 is turned off, leading to non-conductionbetween the wiring 113 and the node ND2.

Note that as illustrated in FIG. 3B, the first terminal and the gate ofthe transistor 103 may be connected to the wiring 116 and the wiring113, respectively. In FIG. 3B, the transistor 103 controls conductionand non-conduction between the wiring 116 and the node ND2. When thewiring 116 and the node ND2 are brought into electrical contact, thepotential of the wiring 116 is supplied to the node ND2, and thepotential of the node ND2 is controlled based on the potential of thewiring 116. When the potential of the wiring 116 is at VH or a highlevel, the potential of the node ND2 is increased. However, since thegate of the transistor 103 is connected to the wiring 113, when thepotential of the node ND2 is increased to a value obtained bysubtracting the threshold voltage of the transistor 103 from thepotential of the wiring 113 at a high level, the transistor 103 isturned off. Thus, the node ND2 is brought into a floating state. In thismanner, the potential of the node ND2 is set to a level at which thetransistor 102 is turned on, and the node ND2 is brought into a floatingstate.

Note that although not illustrated, the first terminal and the gate ofthe transistor 103 may be connected to the wiring 114 and the wiring113, respectively.

Note that although not illustrated, the first terminal and the gate ofthe transistor 103 may be connected to the wiring 113 and the wiring116, respectively.

The transistor 104 controls conduction and non-conduction between thewiring 115 and the node ND2. When the wiring 115 and the node ND2 arebrought into electrical contact, the potential of the wiring 115 issupplied to the node ND2, and the potential of the node ND2 iscontrolled based on the potential of the wiring 115. When the potentialof the wiring 115 is at VL or a low level, the potential of the node ND2is decreased to VL. Thus, the potential of the node ND2 is set to alevel at which the transistor 102 is turned off.

Note that as illustrated in FIG. 4A, the first terminal of thetransistor 104 may be connected to the wiring 113. In FIG. 4A, thetransistor 104 controls conduction and non-conduction between the wiring113 and the node ND2. When the wiring 113 and the node ND2 are broughtinto electrical contact, the potential of the wiring 113 is supplied tothe node ND2, and the potential of the node ND2 is controlled based onthe potential of the wiring 113. When the potential of the wiring 113 isat VL or a low level, the potential of the node ND2 is decreased to VL.Thus, the potential of the node ND2 is set to a level at which thetransistor 102 is turned off.

Note that as illustrated in FIG. 4B, the first terminal of thetransistor 104 may be connected to the wiring 114. In FIG. 4B, thetransistor 104 controls conduction and non-conduction between the wiring114 and the node ND2. When the wiring 114 and the node ND2 are broughtinto electrical contact, the potential of the wiring 114 is supplied tothe node ND2, and the potential of the node ND2 is controlled based onthe potential of the wiring 114. When the potential of the wiring 114 isat VL or a low level, the potential of the node ND2 is decreased to VL.Thus, the potential of the node ND2 is set to a level at which thetransistor 102 is turned off.

Note that as illustrated in FIG. 5A, the gate of the transistor 104 maybe connected to the wiring 111.

Note that as illustrated in FIG. 5B, the gate of the transistor 104 maybe connected to the wiring 112.

The capacitor 105 holds a potential difference between the wiring 112and the node ND1. When the node ND1 is in a floating state, thepotential of the node ND1 is changed based on a change in the potentialof the wiring 112. Accordingly, when the potential of the node ND1 isincreased in accordance with the increase in the potential of the wiring112, the potential of the node ND1 can become higher than the sum of thepotential of the wiring 111 at a high level and the threshold voltage ofthe transistor 101.

Note that as illustrated in FIG. 6A, the capacitor 105 may be omitted.The potential difference between the wiring 112 and the node ND1 is heldby parasitic capacitance between the second terminal and the gate of thetransistor 101.

The capacitor 106 holds a potential difference between the node ND1 andthe node ND2. When the node ND2 is in a floating state, the potential ofthe node ND2 is changed based on a change in the potential of the nodeND1. Accordingly, when the potential of the node ND2 is increased inaccordance with the increase in the potential of the node ND1, thepotential of the node ND2 can become higher than the sum of thepotential of the wiring 113 at a high level and the threshold voltage ofthe transistor 102.

Note that as illustrated in FIG. 6B, the capacitor 106 may be omitted.The potential difference between the node ND1 and the node ND2 is heldby parasitic capacitance between the second terminal and the gate of thetransistor 102.

Note that the transistors 101, 102, 103, and 104, and the capacitors 105and 106 do not necessarily have all the functions described above.

Note that the configurations of the circuit 100 described with referenceto FIG. 1, FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4A and 4B, FIGS. 5Aand 5B, FIGS. 6A and 6B, and the like; and a configuration of thecircuit 100 described without a drawing can be combined as appropriate.

Operation of the semiconductor device in this embodiment is describedusing the configuration in FIG. 1 as an example. Note that oneembodiment of the present invention is not limited to the operationdescribed below.

A timing chart in FIG. 7 shows, as an example, the potential of thewiring 111, the potential of the wiring 113, the potential of the wiring114, on/off of the transistor 104, the potential of the node ND1, thepotential of the node ND2, and the potential of the wiring 112.

For convenience's sake, four periods, that is, a period T1, a period T2,a period T3, and a period T4 are described separately. For example, oneframe period has the periods T1 to T4.

Note that for convenience, in a period T0 just before the period T1, thepotential of the wiring 111 is at a low level, the potential of thewiring 113 is at a low level, the potential of the wiring 114 is at alow level, the potential of the node ND1 is VL, the potential of thenode ND2 is VL, and the potential of the wiring 112 is VL. Since thepotential of the node ND1 is VL, the transistor 101 is off. Furthermore,since the potential of the node ND2 is VL, the transistor 102 is off.

The operation in the period T1 is described with reference to FIG. 8A.The potential of the wiring 111 is kept at a low level, the potential ofthe wiring 113 is kept at a low level, the potential of the wiring 114is changed from a low level to a high level, and the transistor 104 iskept off.

The transistor 103 is turned on because the potential of the wiring 114is set at a high level. Accordingly, the potential of the wiring 114 ata high level is supplied to the node ND2; therefore, the potential ofthe node ND2 is increased from VL.

After that, when the potential of the node ND2 is higher than the sum(VL+Vth102) of the potential of the first terminal of the transistor 102(VL) and the threshold voltage of the transistor 102 (Vth102), thetransistor 102 is turned on. Accordingly, the potential of the wiring113 at a low level is supplied to the node ND1, and thus the potentialof the node ND1 is kept at VL. The transistor 101 is kept off becausethe potential of the node Ni is kept at VL. Thus, the potential of thewiring 112 is kept at VL.

After that, when the potential of the node ND2 is increased to a value(VH−Vth103) obtained by subtracting the threshold voltage of thetransistor 103 (Vth103) from the potential of the gate of the transistor103 (VH), the transistor 103 is turned off. Thus, the node ND2 isbrought into a floating state, and the potential of the node ND2 is keptat VH−Vth103.

Note that in FIG. 2A, since the first terminal of the transistor 103 isconnected to the wiring 116, the potential of the wiring 116 (e.g. VH)is supplied to the node ND2.

The operation in the period T2 is described with reference to FIG. 8B.The potential of the wiring 111 is kept at a low level, the potential ofthe wiring 113 is changed from a low level to a high level, thepotential of the wiring 114 is changed from a high level to a low level,and the transistor 104 is kept off.

Since the potential of the wiring 114 is set at a low level, thetransistor 103 is kept off. Accordingly, the node ND2 is kept in afloating state, and thus the potential of the node ND2 is kept atVH−Vth103. Since the potential of the node ND2 is kept at VH−Vth103, thetransistor 102 is kept on. Accordingly, the potential of the wiring 113at a high level is supplied to the node ND1, and thus the potential ofthe node ND1 is increased from VL. At this time, the capacitor 106 holdsa potential difference between the node ND1 and the node ND2, and thenode ND2 is in a floating state. Accordingly, the potential of the nodeND2 is increased from VH−Vth103 in accordance with the increase in thepotential of the node ND1.

After that, when the potential of the node ND1 is higher than the sum(VL+Vth 101) of the potential of the first terminal of the transistor101 (VL) and the threshold voltage of the transistor 101 (Vth101), thetransistor 101 is turned on. Accordingly, the potential of the wiring111 at a low level is supplied to the wiring 112, and thus the potentialof the wiring 112 is kept at VL.

After that, when the potential of the node ND2 is increased to a level(VH+Vth102+α (α is a positive number)) higher than the sum of thepotential of the first terminal of the transistor 102 (VH) and thethreshold voltage of the transistor 102 (Vth102) in accordance with theincrease in the potential of the node ND1, the potential of the node ND1is increased to VH.

Note that in FIG. 2B, it is preferable that the potential of the wiring114 be kept at a high level in the period T2 in order to keep thetransistor 103 off.

Note that in FIGS. 3A and 3B, the transistor 103 is turned on in theperiod T2 for the first time after the period T0. Specifically, when thepotential of the wiring 113 is set at a high level, the transistor 103is turned on. Accordingly, in FIG. 3A, the potential of the wiring 113at a high level is supplied to the node ND2, and thus the potential ofthe node ND2 is increased from VL. On the other hand, in FIG. 3B, thepotential of the wiring 116 (e.g. VH) is supplied to the node ND2, andthus the potential of the node ND2 is increased from VL. After that,when the potential of the node ND2 is higher than VL+Vth102, thetransistor 102 is turned on. Accordingly, the potential of the wiring113 at a high level is supplied to the node ND1, and thus the potentialof the node ND1 is increased from VL. After that, when the potential ofthe node ND2 is set at VH−Vth103, the transistor 103 is turned off, andthus the node ND2 is brought into a floating state. At this time, thepotential of the node ND1 is increased. The capacitor 106 holds apotential difference between the node ND1 and the node ND2. Accordingly,the potential of the node ND2 is increased from VH−Vth103 in accordancewith the increase in the potential of the node ND1. After that, when thepotential of the node ND1 is higher than VL+Vth101, the transistor 101is turned on. Accordingly, the potential of the wiring 111 at a lowlevel is supplied to the wiring 112, and thus the potential of thewiring 112 is kept at VL. After that, when the potential of the node ND2is increased to VH+Vth102+α in accordance with the increase in thepotential of the node ND1, the potential of the node ND1 is increased toVH. As described above, in FIGS. 3A and 3B, the operation in the periodsT1 and T2 of FIG. 1 can be collectively performed in the period T2.Consequently, the operation speed can be improved.

The operation in the period T3 is described with reference to FIG. 9A.The potential of the wiring 111 is changed from a low level to a highlevel, the potential of the wiring 113 is changed from a high level to alow level, the potential of the wiring 114 is kept at a low level, andthe transistor 104 is changed from an off state to an on state.

Since the potential of the wiring 114 is kept at a low level, thetransistor 103 is kept off. The transistor 104 is turned on.Accordingly, the potential of the wiring 115 is supplied to the nodeND2, and thus the potential of the node ND2 is decreased fromVH+Vth102+α to VL. Since the potential of the node ND2 is set at VL, thetransistor 102 is turned off. Thus, the node ND1 is brought into afloating state, and the potential of the node ND1 is kept at VH. Sincethe potential of the node ND1 is kept at VH, the transistor 101 is kepton. Accordingly, the potential of the wiring 111 at a high level issupplied to the wiring 112, and thus the potential of the wiring 112 isincreased from VL. At this time, the capacitor 105 holds a potentialdifference between the wiring 112 and the node ND1, and the node ND1 isin a floating state. Thus, the potential of the node ND1 is increasedfrom VH in accordance with the increase in the potential of the wiring112.

After that, when the potential of the node ND1 is increased to a level(VH+Vth101+β (β is a positive number)) higher than the sum of thepotential of the first terminal of the transistor 101 (VH) and thethreshold voltage of the transistor 101 (Vth101) in accordance with theincrease in the potential of the wiring 112, the potential of the wiring112 is increased to VH.

Note that in FIG. 4A, since the first terminal of the transistor 104 isconnected to the wiring 113, the potential of the wiring 113 at a lowlevel is supplied to the node ND2. Furthermore, in FIG. 4B, since thefirst terminal of the transistor 104 is connected to the wiring 114, thepotential of the wiring 114 at a low level is supplied to the node ND2.

The operation in the period T4 is described with reference to FIG. 9B.The potential of the wiring 111 is changed from a high level to a lowlevel, the potential of the wiring 113 is kept at a low level, thepotential of the wiring 114 is changed from a low level to a high level,and the transistor 104 is changed from on to off.

Since the potential of the wiring 114 is set at a high level, thetransistor 103 is turned on. Accordingly, the potential of the wiring114 at a high level is supplied to the node ND2, and thus the potentialof the node ND2 is increased. At this time, the potential of the nodeND1 is VH+Vth101+β, and accordingly the transistor 101 is kept on.Accordingly, the potential of the wiring 111 at a low level is suppliedto the wiring 112, and thus the potential of the wiring 112 is decreasedfrom VH to VL.

After that, when the potential of the node ND2 is higher than the sum ofthe potential of the first terminal of the transistor 102 (VL) and thethreshold voltage of the transistor 102 (Vth102), the transistor 102 isturned on. Accordingly, the potential of the wiring 113 at a low levelis supplied to the node ND1, and thus the potential of the node ND1 isdecreased from VH+Vth101+β to VL. The transistor 101 is turned offbecause the potential of the node ND1 is set at VL.

After that, when the potential of the node ND2 is increased to a levelobtained by subtracting the threshold voltage of the transistor 103(Vth103) from the potential of the gate of the transistor 103 (VH), thetransistor 103 is turned off. Thus, the node ND2 is brought into afloating state, and the potential of the node ND2 is kept at VH−Vth103.

Note that since the first terminal of the transistor 103 is connected tothe wiring 116 in FIG. 2A, the potential of the wiring 116 (e.g. VH) issupplied to the node ND2.

The semiconductor device in this embodiment has connection relationswhich enable the above operation, whereby the potential of the node ND2can be set at VH+Vth102+α.

In the semiconductor device in this embodiment, the potential of thenode ND2 is set at VH+Vth102+α, whereby a potential difference betweenthe gate and the source of the transistor 102 can be kept at a levelhigher than the threshold voltage of the transistor 102.

In the semiconductor device in this embodiment, the potential differencebetween the gate and the source of the transistor 102 is kept at a levelhigher than the threshold voltage of the transistor 102, whereby thepotential of the node ND1 can be increased to VH.

In the semiconductor device in this embodiment, the potential differencebetween the gate and the source of the transistor 102 is kept at a levelhigher than the threshold voltage of the transistor 102, whereby thetime required for change in the potential of the node ND1 can beshortened.

In the semiconductor device in this embodiment, the potential of thenode ND1 is increased to VH, whereby the potential difference betweenthe gate and the source of the transistor 101 can be made large.

In the semiconductor device in this embodiment, the potential differencebetween the gate and the source of the transistor 101 is made large,whereby the time required for change in the potential of the wiring 112can be shortened. That is, a signal whose rise time and the fall timeare short can be output to the wiring 112.

In the semiconductor device in this embodiment, the potential differencebetween the gate and the source of each of the transistors 101 and 102is made large, whereby a driving voltage can be made low. This reducesthe power consumption.

In the semiconductor device in this embodiment, the potentialdifferences between the gates and the sources of the transistors 101 and102 are made large, whereby the channel width of each of the transistors101 and 102 can be made small. Thus, the layout area can be decreased.

Since the time required for change in the potential of the node ND1 andthe time required for change in the potential of the wiring 112 can beshortened in the semiconductor device in this embodiment, the operationspeed can be improved.

W/L (W is the channel width and L is the channel length) of each of thetransistors 101, 102, 103, and 104 is described. Note that oneembodiment of the present invention is not limited to the W/L describedbelow.

The transistor 101 drives a potential of the wiring 112, the transistor102 drives a potential of the node ND1, and the transistors 103 and 104drive a potential the node ND2. The load of the wiring 112 is oftenlarger than any of the loads of the node ND1 and the node ND2.Therefore, W/L of the transistor 101 is preferably larger than that ofthe transistors 102, 103, and 104. Alternatively, the W/L of thetransistor 101 is preferably the largest among the transistors includedin the circuit 100. Further alternatively, the W/L of the transistor 101is preferably the largest among transistors provided over the samesubstrate as the circuit 100. Note that a transistor whose W/L is thesame or substantially the same as that of the transistor 101 may beprovided. In this manner, the drive capability of the transistor 101 canbe improved, which enables the load of the wiring 112 to be made large.Furthermore, the transistors 102, 103, and 104 can be downsized, leadingto reduction in a layout area.

Even when the potential of the node ND1 is increased, the potentialdifference between the gate and the source of the transistor 102 can bekept at a level of greater than or equal to the threshold voltage of thetransistor 102, and thus the W/L of the transistor 102 can be made low.On the other hand, when the potential of the node ND2 is increased, apotential difference between the gate and the source of the transistor103 is decreased gradually; therefore, the W/L of the transistor 103 ispreferably high. Therefore, the W/L of the transistor 103 is preferablyhigher than that of the transistor 102. In this manner, the drivecapability of the transistor 103 can be improved, and thus the timerequired for change in the potential of the node ND2 can be shortened.Furthermore, reduction in size of the transistor 102 can be achieved,leading to reduction in a layout area. But it is also possible to makethe W/L of the transistor 102 higher than that of the transistor 103.

A transistor which can be additionally included in any of theconfigurations of the circuit 100 described with reference to FIG. 1,FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4A and 4B, FIGS. 5A and 5B, andFIGS. 6A and 6B; the configuration of the circuit 100 described withouta drawing; and a configuration obtained by combining them is described.Note that one embodiment of the present invention is not limited to aconfiguration described below.

A transistor 107 may be additionally included in any of the aboveconfigurations of the circuit 100. FIG. 10A illustrates a case where thetransistor 107 is additionally included in the circuit 100 in FIG. 1. Afirst terminal and a second terminal of the transistor 107 are connectedto a wiring 115B and the wiring 112, respectively. The potential of thewiring 115B is preferably VL. Note that the potential of the wiring 115Bmay have a high level and a low level. The transistor 107 controlsconduction and non-conduction between the wiring 115B and the wiring112. When the transistor 107 is turned on, the wiring 115B and thewiring 112 are brought into electrical contact, and the potential of thewiring 112 is controlled based on the potential of the wiring 115B. Whenthe potential of the wiring 115B is VL or at a low level, the potentialof the wiring 112 is VL. Furthermore, the transistor 107 preferably hasthe same polarity as the transistors 101 to 104.

In the period T1, the transistor 107 is turned on. Accordingly, thepotential of the wiring 115B is supplied to the wiring 112, and thus thepotential of the wiring 112 is set at VL. Note that in the period T1,the transistor 107 may be off.

In the period T2, the transistor 107 is turned on. Accordingly, thepotential of the wiring 115B is supplied to the wiring 112, and thus thepotential of the wiring 112 is set at VL. Note that in the period T2,the transistor 107 may be off.

In the period T3, the transistor 107 is turned off.

In the period T4, the transistor 107 is turned on. Accordingly, thepotential of the wiring 115B is supplied to the wiring 112, and thus thepotential of the wiring 112 is set at VL. Note that in the period T4,the transistor 107 may be off.

In FIG. 10A, the circuit 100 includes the transistor 107, whereby thewiring 112 is prevented from being brought into a floating state, whichcan stabilize the potential of the wiring 112. Accordingly, malfunctionscan be prevented.

Note that the first terminal of the transistor 107 may be connected tothe wiring 111, 113, 114, or 115. A gate of the transistor 107 may beconnected to the wiring 113 or 114.

A transistor 108 may be additionally included in the circuit 100 havingany of the above configurations. FIG. 10B illustrates a case where thetransistor 108 is additionally included in the circuit 100 in FIG. 1. Afirst terminal and a second terminal of the transistor 108 are connectedto a wiring 115C and the node ND1, respectively. The potential of thewiring 115C is preferably VL. Note that the potential of the wiring 115Cmay have a high level and a low level. The transistor 108 controlsconduction and non-conduction between the wiring 115C and the node ND1.When the transistor 108 is turned on, the wiring 115C and the node ND1are brought into electrical contact, and the potential of the node ND1is controlled based on the potential of the wiring 115C. When thepotential of the wiring 115C is VL or at a low level, the potential ofthe node ND1 is set at VL. Thus, the potential of the node ND1 is set toa level at which the transistor 101 is turned off. Furthermore, thetransistor 108 preferably has the same polarity as the transistors 101to 104.

In the period T1, the transistor 108 is turned on. Accordingly, thepotential of the wiring 115C is supplied to the node ND1, and thus thepotential of the node ND1 is set at VL. Note that in the period T1, thetransistor 108 may be off.

In the period T2, the transistor 108 is turned off.

In the period T3, the transistor 108 is turned off.

In the period T4, the transistor 108 is turned on. Accordingly, thepotential of the wiring 115C is supplied to the node ND1, and thus thepotential of the node ND1 is set at VL. Note that in the period T4, thetransistor 108 may be off.

In FIG. 10B, the circuit 100 includes the transistor 108, whereby thenode ND1 is prevented from being brought into a floating state, whichcan stabilize the potential of the node ND1. Accordingly, malfunctionscan be prevented.

Note that the first terminal of the transistor 108 may be connected tothe wirings 111, 113, 114, or 115. A gate of the transistor 108 may beconnected to the wiring 114.

Note that in the case where the transistors 107 and 108 are bothadditionally included in the circuit 100, the gate of the transistor 107may be connected to the gate of the transistor 108. Alternatively, thefirst terminal of the transistor 107 may be connected to the firstterminal of the transistor 108.

In the circuit 100 having any of the above configurations, thetransistors 107 and/or 108 may be additionally included, and atransistor 109 and a transistor 110 may be additionally included. FIG.11A illustrates a case where the transistors 109 and 110 areadditionally included in addition to the transistors 107 and 108 in thecircuit 100 illustrated in FIG. 1. The first terminal of the transistor109 is connected to the wiring 116, the second terminal of thetransistor 109 is connected to the gates of the transistors 107 and 108,and the gate of the transistor 109 is connected to the wiring 114. Afirst terminal, a second terminal, and a gate of the transistor 110 areconnected to the wiring 114, the gates of the transistors 107 and 108,and the node ND1, respectively. Note that the gate of the transistor107, the gate of the transistor 108, the second terminal of thetransistor 109, or the second terminal of the transistor 110 is referredto as a node ND3. The transistor 109 controls conduction andnon-conduction between the wiring 116 and the node ND3. When thetransistor 109 is turned on, the wiring 116 and the node ND3 are broughtinto electrical contact, and the potential of the node ND3 is controlledbased on the potential of the wiring 116. When the potential of thewiring 116 is VH or at a high level, the potential of the node ND3 isincreased. However, since the gate of the transistor 109 is connected tothe wiring 114, when the potential of the node ND3 is increased to alevel obtained by subtracting the threshold voltage of the transistor109 from the potential of the wiring 114 at a high level, the transistor109 is turned off. Thus, the node ND3 is brought into a floating state.In this manner, the potential of the node ND3 is set to a level at whichthe transistor 107 or the transistor 108 is turned on, and the node ND3is brought into a floating state. The transistor 110 controls conductionand non-conduction between the wiring 114 and the node ND3. When thetransistor 110 is turned on, the wiring 114 and the node ND3 are broughtinto electrical contact, and the potential of the node ND3 is controlledbased on the potential of the wiring 114. When the potential of thewiring 114 is at a low level, the potential of the node ND3 is decreasedto VL. In this manner, the potential of the node ND3 is set to a levelat which the transistor 107 or 108 is turned off. The transistors 109and 110 preferably have the same polarity as the transistors 101 to 104.

Since the wiring 114 is set at a high level in the period T1, thetransistor 109 is turned on. Since the potential of the node ND1 is setat VL, the transistor 110 is turned off. Accordingly, the potential ofthe wiring 116 is supplied to the node ND3, and thus the potential ofthe node ND3 is increased from VL. After that, when the potential of thenode ND3 is higher than the sum of the potential of the first terminalof the transistor 107 (VL) and the threshold voltage of the transistor107 (Vth107), the transistor 107 is turned on. Furthermore, when thepotential of the node ND3 is higher than the sum of the potential of thefirst terminal of the transistor 108 (VL) and the threshold voltage ofthe transistor 108 (Vth108), the transistor 108 is turned on. Afterthat, when the potential of the node ND3 is set to a level obtained bysubtracting the threshold voltage of the transistor 109 (Vth109) fromthe potential of the gate of the transistor 109 (VH), the transistor 109is turned off. Thus, the node ND3 is brought into a floating state, andthe potential of the node ND3 is kept at VH−Vth109.

Since the potential of the wiring 114 is set at a low level in theperiod T2, the transistor 109 is turned off. Furthermore, when thepotential of the node ND1 is higher than the sum of the potential of thefirst terminal of the transistor 110 (VL) and the threshold voltage ofthe transistor 110 (Vth110), the transistor 110 is turned on.Accordingly, the potential of the wiring 114 at a low level is suppliedto the node ND3, and thus the potential of the node ND3 is decreasedfrom VH−Vth109 to VL. Accordingly, the transistors 107 and 108 areturned off.

Since the potential of the wiring 114 is kept at a low level in theperiod T3, the transistor 109 is kept off. Furthermore, since thepotential of the node ND1 is set at VH+Vth110+β, the transistor 110 iskept on. Accordingly, the potential of the wiring 114 at a low level issupplied to the node ND3, and thus the potential of the node ND3 is keptat VL. Accordingly, the transistors 107 and 108 are kept off.

Since the potential of the wiring 114 is set at a high level in theperiod T4, the transistor 109 is turned on. Since the potential of thenode ND1 is set at VL, the transistor 110 is turned off. Accordingly,the potential of the wiring 116 is supplied to the node ND3, and thusthe potential of the node ND3 is increased from VL. After that, when thepotential of the node ND3 is higher than the sum of the potential of thefirst terminal of the transistor 107 (VL) and the threshold voltage ofthe transistor 107 (Vth107), the transistor 107 is turned on.Furthermore, when the potential of the node ND3 is higher than the sumof the potential of the first terminal of the transistor 108 (VL) andthe threshold voltage of the transistor 108 (Vth108), the transistor 108is turned on.

In FIG. 11A, the circuit 100 includes the transistors 109 and 110,whereby a signal for controlling the transistors 107 or 108 can begenerated in the circuit 100. Accordingly, the number of signals can bereduced.

Note that as illustrated in FIG. 11B, the gate of the transistor 109 andthe first terminal of the transistor 110 may be connected to a wiring117. The potential of the wiring 117 has a high level (e.g. VH) and alow level (e.g. VL).

Note that the gate of the transistor 109 may be connected to the wiring117, and the first terminal of the transistor 110 may be connected tothe wiring 114. Alternatively, the gate of the transistor 109 may beconnected to the wiring 114, and the first terminal of the transistor110 may be connected to the wiring 117.

Note that a configuration in which the second terminals of thetransistors 109 and 110 are connected to the gate of the transistor 107and are not connected to the gate of the transistor 108 may be employed.Alternatively, a configuration in which the second terminals of thetransistors 109 and 110 are connected to the gate of the transistor 108and are not connected to the gate of the transistor 107 may be employed.

Note that the first terminal of the transistor 110 may be connected tothe wirings 115, 115B, 115C, or 117.

In the circuit 100 having any of the above configurations, a transistor121 may be additionally included. FIG. 12A illustrates a case where thetransistor 121 is additionally included in the circuit 100 in FIG. 1. Afirst terminal, a second terminal, and a gate of the transistor 121 areconnected to the wiring 112, the node ND1, and the wiring 111,respectively. The transistor 121 controls conduction and non-conductionbetween the wiring 112 and the node ND1. When the transistor 121 isturned on, the wiring 112 and the node ND1 are brought into electricalcontact. For example, in the case where the potential of the wiring 112is increased from VL and the potential of the node ND1 is increased fromVH as in the period T3, an increase in the potential of the node ND1 issuppressed, and the time required for change in the potential of thewiring 112 is shortened. Note that since the gate of the transistor 121is connected to the wiring 111, when the potential of the wiring 112 isset to a level obtained by subtracting the threshold voltage of thetransistor 121 from the potential of the wiring 111 at a high level, thetransistor 121 is turned off. The transistor 121 preferably has the samepolarity as the transistors 101 to 104.

In the period T1, the wiring 111 is at a low level, and thus thetransistor 121 is turned off.

In the period T2, the wiring 111 is at a low level, and thus thetransistor 121 is turned off.

In the period T3, the wiring 111 is set at a high level, and thus thetransistor 121 is turned on. Note that when the potential of the wiring112 is increased to a level obtained by subtracting the thresholdvoltage of the transistor 121 (Vth 121) from the potential of the gateof the transistor 121 (VH), the transistor 121 is turned off.

Since the wiring 111 is set at a low level in the period T4, thetransistor 121 is turned off.

In FIG. 12A, the circuit 100 includes the transistor 121, whereby thepotential of the node ND1 can be prevented from being too high. Thus,deterioration of the transistor connected to the node ND1 can besuppressed, and damage of the transistor can be prevented, for example.

In the circuit 100 having any of the above configurations, a transistor122 may be additionally included. FIG. 12B illustrates a case where thetransistor 122 is additionally included in the circuit 100 in FIG. 1. Afirst terminal, a second terminal, and a gate of the transistor 122 areconnected to the node ND1, the node ND2, and the wiring 113,respectively. The transistor 122 controls conduction and non-conductionbetween the node ND1 and the node ND2. When the transistor 122 is turnedon, the node ND1 and the node ND2 are brought into electrical contact.For example, in the case where the potential of the node ND1 isincreased from VL and the potential of the node ND2 is increased fromVH−Vth103 as in the period T2, an increase in the potential of the nodeND2 is suppressed, and the time required for change in the potential ofthe node ND1 is shortened. Note that since the gate of the transistor122 is connected to the wiring 113, when the potential of the node ND1is set to a level obtained by subtracting the threshold voltage of thetransistor 122 from the potential of the wiring 113 at a high level, thetransistor 122 is turned off. The transistor 122 preferably has the samepolarity as the transistors 101 to 104.

Since the wiring 113 is at a low level in the period T1, the transistor122 is turned off.

Since the wiring 113 is set at a high level in the period T2, thetransistor 122 is turned on. Note that when the potential of the nodeND1 is increased to a level obtained by subtracting the thresholdvoltage of the transistor 122 (Vth 122) from the potential of the gateof the transistor 122 (VH), the transistor 122 is turned off.

Since the wiring 113 is set at a low level in the period T3, thetransistor 122 is turned off.

Since the wiring 113 is set at a low level in the period T4, thetransistor 122 is turned off.

In FIG. 12B, the circuit 100 includes the transistor 122, whereby thepotential of the node ND2 can be prevented from being too high. Thus,deterioration of the transistor connected to the node ND2 can besuppressed, and damage of the transistor can be prevented, for example.

In the circuit 100 having any of the above configurations, a transistor123 may be additionally included. FIG. 13A illustrates a case where thetransistor 123 is additionally included in the circuit 100 in FIG. 1. Afirst terminal and a second terminal of the transistor 123 are connectedto the wiring 111 and the node ND1, respectively. The transistor 123controls conduction and non-conduction between the wiring 111 and thenode ND1. When the transistor 123 is turned on, the wiring 111 and thenode ND1 are brought into electrical contact, and the potential of thewiring 111 is supplied to the node ND1. When the potential of the wiring111 is at a low level, the potential of the node ND1 is set at VL. Thus,the potential of the node ND1 is set to a level at which the transistor101 is turned off. The transistor 123 preferably has the same polarityas the transistors 101, 102, 103, and 104.

In the period T0, the transistor 123 is turned on. Accordingly, thepotential of the wiring 111 at a low level is supplied to the node ND1,and thus the potential of the node ND1 is set at VL.

In the periods T1, T2, T3, and T4, the transistor 123 is turned off.

In FIG. 13A, the circuit 100 includes the transistor 123, whereby thepotential of the node ND1 can be set at VL. Accordingly, malfunctionscan be prevented.

In the circuit 100 having any of the above configurations, a transistor124 may be additionally included. FIG. 13B illustrates a case where thetransistor 124 is additionally included in the circuit 100 in FIG. 1. Afirst terminal and a second terminal of the transistor 124 are connectedto the wiring 113 and the node ND2, respectively. The transistor 124controls conduction and non-conduction between the wiring 113 and thenode ND2. When the transistor 124 is turned on, the wiring 113 and thenode ND2 are brought into electrical contact, and the potential of thewiring 113 is supplied to the node ND2. When the potential of the wiring113 is at a low level, the potential of the node ND2 is set at VL. Thus,the potential of the node ND2 is set to a level at which the transistor102 is turned off. The transistor 124 preferably has the same polarityas the transistors 101, 102, 103, and 104.

In the period T0, the transistor 124 is turned on. Accordingly, thepotential of the wiring 113 at a low level is supplied to the node ND2,and thus the potential of the node ND2 is set at VL.

In the periods T1, T2, T3, and T4, the transistor 124 is turned off.

In FIG. 13B, the circuit 100 includes the transistor 124, whereby thepotential of the node ND2 can be set to VL. Accordingly, malfunctionscan be prevented.

Note that in the case where both of the transistors 123 and 124 areadditionally included in the circuit 100, a gate of the transistor 123may be connected to a gate of the transistor 124.

Note that the circuit 100 described with reference to FIG. 1, FIGS. 2Aand 2B, FIGS. 3A and 3B, FIGS. 4A and 4B, FIGS. 5A and 5B, FIGS. 6A and6B, FIGS. 10A and 10B, FIGS. 11A and 11B, FIGS. 12A and 12B, FIGS. 13Aand 13B, and the like; and the circuit 100 described without a drawingcan be combined as appropriate.

FIG. 14A illustrates a configuration where a configuration in which thefirst terminal and the gate of the transistor 103 are connected to thewiring 113 (see FIG. 3A) and a configuration in which the gate of thetransistor 104 is connected to the wiring 111 (see FIG. 5A) arecombined.

FIG. 14B illustrates a configuration where a configuration in which thegate of the transistor 104 is connected to the wiring 111 (see FIG. 5A),a configuration in which the transistor 108 is additionally included(see FIG. 10B), and a configuration in which the transistors 109 and 110are additionally included (see FIG. 11B) are combined.

This embodiment can be combined with any of the other embodiments asappropriate. Thus, content (or may be part of the content) described inthis embodiment may be applied to, combined with, or replaced bydifferent content (or may be part of the different content) described inthe embodiment and/or content (or may be part of the content) describedin one or more different embodiments. Note that in each embodiment, acontent described in the embodiment is a content described withreference to a variety of diagrams or a content described with a textdescribed in this specification. In addition, by combining a diagram (orpart thereof) described in one embodiment with another part of thediagram, a different diagram (or part thereof) described in the sameembodiment, and/or a diagram (or part thereof) described in another orother embodiments, much more diagrams can be formed. This applies alsoto other embodiments.

Embodiment 2

In this embodiment, a semiconductor device of one embodiment of thepresent invention is described.

A structure of a semiconductor device of one embodiment of the presentinvention is described with reference to FIG. 15. Note that oneembodiment of the present invention is not limited to the structuredescribed below.

A semiconductor device illustrated in FIG. 15 includes a circuit 200.The circuit 200 has a function of controlling potentials of N (N is anatural number of 3 or more) wirings 211 (also referred to as wirings211[1] to [N]) based on potentials of a wirings 212, 213, 214, and 215.The circuit 200 outputs signals based on the potentials of the wirings212, 213, 214, and 215 to the corresponding wirings 211[1] to [N]. Thepotentials of the wirings 211[1] to [A] are controlled with the signals.

Specifically, based on the potentials of the wirings 212, 213, 214, and215, the circuit 200 has a function of sequentially making thepotentials of the wirings 211[1] to [N] active, that is, a function ofsequentially setting the potentials of the wirings 211[1] to [N] at ahigh level or a low level. FIG. 16 is a timing chart in the case ofsequentially setting the potentials of the wirings 211[1] to [N] at ahigh level based on the potentials of the wirings 212, 213, 214, and215. As described above, the circuit 200 has a function of a shiftregister.

Note that the potentials of the wirings 212, 213, 214, and 215 arecontrolled by signals, voltage, or the like being input to thecorresponding wirings. For example, a signal CK1 is input to the wiring212, a signal CK2 is input to the wiring 213, a signal CK3 is input tothe wiring 214, a signal SP is input to the wiring 215, and signalsOUT[1] to [N] are output to the wirings 211[1] to [N]. That is, thesignals OUT[1] to [N] each have a value based on the signals CK1, CK2,and CK3, and the signal SP. As the signals CK1, CK2, and CK3, clocksignals whose phases are different are used. As the signal SP, a startpulse is used.

The circuit 200 includes N circuits 201 (also referred to as circuits201[1] to [N]). Each of the circuits 201[1] to [N] corresponds to thecircuit 100 described in Embodiment 1. In FIG. 15, the circuit 100illustrated in FIG. 5A is used as each of the circuits 201[1] to [N].

In the circuit 201[2m+1] (m is 0 or a positive integer), the firstterminal of the transistor 101 and the gate of the transistor 104 areconnected to the wiring 214. Accordingly, the wiring 214 corresponds tothe wiring 111. The second terminal of the transistor 101 is connectedto the wiring 211[2m+1]. Accordingly, the wiring 211[2m+1] correspondsto the wiring 112. The first terminal of the transistor 102 is connectedto the wiring 215 or the wiring 211[2m]. Accordingly, the wiring 215 orthe wiring 211[2m] corresponds to the wiring 113. The first terminal andthe gate of the transistor 103 are connected to the wiring 212.Accordingly, the wiring 212 corresponds to the wiring 114. The firstterminal of the transistor 104 is connected to the wiring 213.Accordingly, the wiring 213 corresponds to the wiring 115.

In the circuit 201[2m+2], the first terminal of the transistor 101 andthe gate of the transistor 104 are connected to the wiring 212.Accordingly, the wiring 212 corresponds to the wiring 111. The secondterminal of the transistor 101 is connected to the wiring 211[2m+2].Accordingly, the wiring 211[2m+2] corresponds to the wiring 112. Thefirst terminal of the transistor 102 is connected to the wiring211[2m+1]. Accordingly, the wiring 211[2m+1] corresponds to the wiring113. The first terminal and the gate of the transistor 103 are connectedto the wiring 213. Accordingly, the wiring 213 corresponds to the wiring114. The first terminal of the transistor 104 is connected to the wiring214. Accordingly, the wiring 214 corresponds to the wiring 115.

In the circuit 201[2m+3], the first terminal of the transistor 101 andthe gate of the transistor 104 are connected to the wiring 213.Accordingly, the wiring 213 corresponds to the wiring 111. The secondterminal of the transistor 101 is connected to the wiring 211[2m+3].Accordingly, the wiring 211[2m+3] corresponds to the wiring 112. Thefirst terminal of the transistor 102 is connected to the wiring211[2m+2]. Accordingly, the wiring 211[2m+2] corresponds to the wiring113. The first terminal and the gate of the transistor 103 are connectedto the wiring 214. Accordingly, the wiring 214 corresponds to the wiring114. The first terminal of the transistor 104 is connected to the wiring212. Accordingly, the wiring 212 corresponds to the wiring 115.

Note that as illustrated in FIG. 17, in each of the circuits 201[1] to[N], the first terminal of the transistor 104 may be connected to awiring 216. The wiring 216 corresponds to the wiring 115. The wiring 216may be supplied with a voltage VSS. The voltage VSS has a value whichcorresponds to (which is the same or substantially the same as) lowlevels of the signals CK1, CK2, and CK3, and the signal SP.

Note that as illustrated in FIG. 18, in the circuit 201[2m+1], the firstterminal and the gate of the transistor 103 may be connected to thewiring 213. In the circuit 201[2m+2], the first terminal and the gate ofthe transistor 103 may be connected to the wiring 214. In the circuit201[2m+3], the first terminal and the gate of the transistor 103 may beconnected to the wiring 212. That is, in the circuit 201[i] (i is any of2 to N), the first terminal and the gate of the transistor 103 may beconnected to any of the wirings 212, 213, and 214 to which the firstterminal of the transistor 101 in the circuit 201[i−1] is connected.

Note that in the case of employing the circuit 100 in which the firstterminal or the gate of the transistor 103 is connected to the wiring116 in each of the circuits 201[1] to [N] (see FIG. 2A, 2B, and FIG. 3B,for example), a wiring to which the first terminal or the gate of thetransistor 103 in the circuits 201[1] to [N] is connected may beadditionally provided.

This embodiment can be combined with any of the other embodiments asappropriate. Thus, content (or may be part of the content) described inthis embodiment may be applied to, combined with, or replaced bydifferent content (or may be part of the different content) described inthe embodiment and/or content (or may be part of the content) describedin one or more different embodiments. Note that in each embodiment, acontent described in the embodiment is a content described withreference to a variety of diagrams or a content described with a textdescribed in this specification. In addition, by combining a diagram (orpart thereof) described in one embodiment with another part of thediagram, a different diagram (or part thereof) described in the sameembodiment, and/or a diagram (or part thereof) described in another orother embodiments, much more diagrams can be formed. This applies alsoto other embodiments.

Embodiment 3

In this embodiment, a display device of one embodiment of the presentinvention is described.

A structure of a display device of one embodiment of the presentinvention is described with reference to FIG. 19. Note that oneembodiment of the present invention is not limited to the structuredescribed below.

A display device illustrated in FIG. 19 includes a pixel portion 301, ascan line driver circuit 302, and a signal line driver circuit 303.

In the pixel portion 301, N scan lines GL (also referred to as scanlines GL [1] to [N]) and M (M is a natural number of two or more) signallines SL (also referred to as signal lines SL [1] to [M]) are providedso as to intersect with each other. A pixel 310 is provided at eachintersection.

The pixel 310 includes at least a display element and a transistor.Examples of a display element include a light-emitting element and aliquid crystal element. An example of a light-emitting element includesan EL element.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ a variety of modes or caninclude a variety of elements. A display element, a display device, alight-emitting element, or a light-emitting device includes, forexample, at least one of an electroluminescence (EL) element (e.g., anEL element including organic and inorganic materials, an organic ELelement, or an inorganic EL element), an LED (e.g., a white LED, a redLED, a green LED, or a blue LED), a transistor (a transistor that emitslight depending on current), an electron emitter, a liquid crystalelement, electronic ink, an electrophoretic element, a grating lightvalve (GLV), a plasma display panel (PDP), a display element using microelectro mechanical system (MEMS), a digital micromirror device (DMD), adigital micro shutter (DMS), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, and a display element including a carbon nanotube. Other thanthe above, a display medium whose contrast, luminance, reflectance,transmittance, or the like is changed by electrical or magnetic actionmay be included. Note that examples of display devices having ELelements include an EL display. Display devices having electron emittersinclude a field emission display (FED), an SED-type flat panel display(SED: surface-conduction electron-emitter display), and the like.Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). An example of a display device including electronicink or electrophoretic elements is electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption. Note that in the case of using an LED,graphene or graphite may be provided under an electrode or a nitridesemiconductor of the LED. Graphene or graphite may be a multilayer filmin which a plurality of layers are stacked. As described above,provision of graphene or graphite enables easy formation of a nitridesemiconductor film thereover, such as an n-type GaN semiconductor layerincluding crystals. Furthermore, a p-type GaN semiconductor layerincluding crystals or the like can be provided thereover, and thus theLED can be formed. Note that an AlN layer may be provided between then-type GaN semiconductor layer including crystals and graphene orgraphite. The GaN semiconductor layers included in the LED may be formedby MOCVD. Note that when the graphene is provided, the GaN semiconductorlayers included in the LED can also be formed by a sputtering method.

The scan line driver circuit 302 has a function of controllingpotentials of the scan lines GL [1] to [N]. The scan line driver circuit302 outputs a scan signal to each of the scan lines GL[1] to [N]. Eachof the potentials of the scan lines GL[1] to [N] is controlled by thescan signal. The scan line driver circuit 302 can include the circuit100 described in Embodiment 1 or the circuit 200 described in Embodiment2. In such a case, each of the scan lines GL[1] to [N] corresponds tothe wiring 112, for example. Alternatively, the scan lines GL[1] to [N]correspond to the wiring 211[1] to [N]. Signals which control the scanline driver circuit 302 (a clock signal, a start pulse, and the like, orthe signals CK1, CK2, and CK3, and the signal SP, and the like) aresupplied from the circuit 304.

The signal line driver circuit 303 has a function of controlling thepotentials or the current of the signal lines SL[1] to [M]. The signalline driver circuit 303 outputs a video signal to each of the signallines SL[1] to [M]. Each of the potentials of the signal lines SL[1] to[M] is controlled with the video signal. The signal line driver circuit303 can include the circuit 100 described in Embodiment 1 or the circuit200 described in Embodiment 2. Signals which control the signal linedriver circuit 303 (e.g., a clock signal, a start pulse, a video signal,and the like) are supplied from the circuit 304.

Note that the circuit 304 serves as a timing controller for supplyingsignals to the scan line driver circuit 302 and the signal line drivercircuit 303. The circuit 304 may apply a voltage to the scan line drivercircuit 302 and the signal line driver circuit 303. In such a case, thecircuit 304 serves as a power supply circuit.

Note that the scan line driver circuit 302 is operated at a lower speedthan the signal line driver circuit 303. Accordingly, a transistorincluded in the scan line driver circuit 302 preferably includes anoxide semiconductor, a polycrystalline silicon, or an amorphous siliconin a channel formation region. A transistor included in the signal linedriver circuit 303 preferably includes single crystal silicon in achannel formation region. Accordingly, it is preferable that the pixelportion 301 and the scan line driver circuit 302 be provided over thesame substrate and the signal line driver circuit 303 be provided overanother substrate. Note that the pixel portion 301, the scan line drivercircuit 302, and the signal line driver circuit 303 may be provided overthe same substrate.

Note that the circuit 100 described in Embodiment 1 or the circuit 200described in Embodiment 2 is used in the scan line driver circuit 302,whereby all the transistors included in the scan line driver circuit 302can have the same polarity. Accordingly, in the case where the pixelportion 301 and the scan line driver circuit 302 are provided over thesame substrate, all the transistors provided over the substratepreferably have the same polarity.

Note that the circuit 100 described in Embodiment 1 or the circuit 200described in Embodiment 2 is used in the scan line driver circuit 302,whereby a layout area of the scan line driver circuit 302 can bereduced. Accordingly, the resolution of the pixel 310 can be improved.Furthermore, the frame can be reduced.

This embodiment can be combined with any of the other embodiments asappropriate. Thus, content (or may be part of the content) described inthis embodiment may be applied to, combined with, or replaced bydifferent content (or may be part of the different content) described inthe embodiment and/or content (or may be part of the content) describedin one or more different embodiments. Note that in each embodiment, acontent described in the embodiment is a content described withreference to a variety of diagrams or a content described with a textdescribed in this specification. In addition, by combining a diagram (orpart thereof) described in one embodiment with another part of thediagram, a different diagram (or part thereof) described in the sameembodiment, and/or a diagram (or part thereof) described in another orother embodiments, much more diagrams can be formed. This applies alsoto other embodiments.

Embodiment 4

In this embodiment, a structure of the semiconductor device described inEmbodiment 1 is described.

FIG. 20 is a top view of the semiconductor device illustrated in FIG.5A. FIG. 23 is a cross-sectional view taken along line A-B of the topview of FIG. 20. Note that one embodiment of the present invention isnot limited to the structure described below.

The semiconductor device illustrated in FIG. 20 includes conductivelayers 401A to 401D, semiconductor layers 402A to 402D, conductivelayers 403A to 403I, and an insulating layer 404. FIG. 21 illustratesonly the conductive layers 401A to 401D. FIG. 22 illustrates only theconductive layers 403A to 403I. Note that the X direction issubstantially perpendicular to the Y direction. Alternatively, the Xdirection is a direction intersecting with the Y direction.

The insulating layer 404 includes a region serving as a gate insulatinglayer of the transistor 101, a region serving as a gate insulating layerof the transistor 102, a region serving as a gate insulating layer ofthe transistor 103, and a region serving as a gate insulating layer ofthe transistor 104. Furthermore, the insulating layer 404 includes aregion interposed between the conductive layer 401A and thesemiconductor layer 402A, a region interposed between the conductivelayer 401B and the semiconductor layer 402B, a region interposed betweenthe conductive layer 401C and the semiconductor layer 402C, and a regioninterposed between the conductive layer 401D and the semiconductor layer402D. Note that black circles in the figures indicate contact holes inthe insulating layer 404.

As the insulating layer 404, an insulating layer including at least oneof the following films formed by a plasma enhanced chemical vapordeposition (PECVD) method, a sputtering method, or the like can be used:a silicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, a silicon nitride film, an aluminum oxide film, a hafnium oxidefilm, an yttrium oxide film, a zirconium oxide film, a gallium oxidefilm, a tantalum oxide film, a magnesium oxide film, a lanthanum oxidefilm, a cerium oxide film, and a neodymium oxide film.

The conductive layers 401A to 401D are in the same layer. Alternatively,the conductive layers 401A to 401D include the same material. Furtheralternatively, the conductive layers 401A to 401D are formed through astep of processing the same conductive film.

The conductive layers 401A to 401D can each be formed using a metalelement selected from chromium (Cr), copper (Cu), aluminum (Al), gold(Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta), titanium(Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), and cobalt(Co); an alloy including any of these metal element as its component; analloy including a combination of any of these elements; or the like.

In addition, the conductive layers 401A to 401D may have a single-layerstructure or a layered structure of two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film or a tungstennitride film, a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in this order, and thelike can be given. Alternatively, an alloy film or a nitride film whichcontains aluminum and one or more elements selected from titanium,tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may beused.

Alternatively, the conductive layers 401A to 401D can be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be usedfor the conductive layers 401A to 401D. Use of a Cu—X alloy film enablesthe manufacturing cost to be reduced because wet etching process can beused in the processing.

The conductive layer 401A includes a region serving as a gate electrodeof the transistor 101 and a region serving as a second electrode of thecapacitor 105. The conductive layer 401A also includes an opening 401A1and an opening 401A2. Each of the opening 401A1 and the opening 401A2 islong along substantially the Y direction.

The conductive layer 401B includes a region serving as a gate electrodeof the transistor 102 and a region serving as a second electrode of thecapacitor 106. The conductive layer 401B also includes an opening 401B1.The opening 401B1 is long along substantially the Y direction.

The conductive layer 401C includes a region serving as a gate electrodeof the transistor 103.

The conductive layer 401D includes a region serving as a gate electrodeof the transistor 104.

Note that the area of the conductive layer 401A is larger than each ofthe areas of the conductive layers 401B, 401C, and 401D. Furthermore,the area of the conductive layer 401B is larger than each of the areasof the conductive layers 401C and 401D.

Note that each of the areas of the openings 401A1 and 401A2 is largerthan the area of the opening 401B1. Each of the widths of the openings401A1 and 401A2 is larger than the width of the opening 401B1. Each ofthe lengths in a long length direction of the openings 401A1 and 401A2is larger than the length in the long length direction of the opening401B1.

Note that three or more openings may be provided in the conductive layer401A, and two or more openings may be provided in the conductive layer401B. Note that the number of openings included in the conductive layer401A is preferably larger than that in the conductive layer 401B.

The semiconductor layers 402A to 402D are in the same layer.Alternatively, the semiconductor layers 402A to 402D include the samematerial. Further alternatively, the semiconductor layers 402A to 402Dare formed through a step of processing the same semiconductor film.

For the semiconductor layers 402A to 402D, a single crystalsemiconductor or a non-single-crystal semiconductor can be used.Examples of a non-single-crystal semiconductor includenon-single-crystal silicon and non-single-crystal germanium. Examples ofnon-single-crystal silicon include amorphous silicon, microcrystallinesilicon, and polycrystalline silicon. Examples of non-single-crystalgermanium include amorphous germanium, microcrystalline germanium, andpolycrystalline germanium.

It is particularly preferable that an oxide semiconductor film beprocessed to form the semiconductor layers 402A to 402D. For an oxidesemiconductor film, an In-M oxide (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd,or Hf) or an In-M-Zn oxide can be used. It is particularly preferable touse In-M-Zn oxide for the oxide semiconductor film. In the case wherethe oxide semiconductor film is an In-M-Zn oxide, it is preferable thatthe atomic ratio of metal elements of a sputtering target used forforming a film of the In-M-Zn oxide satisfy In M and Zn M As the atomicratio of metal elements of such a sputtering target, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, In:M:Zn=2:1:3, In:M:Zn=3:1:2, and In:M:Zn=4:2:4.1 arepreferable. When the oxide semiconductor film is an In-M-Zn oxide, atarget including polycrystalline In-M-Zn oxide is preferably used as asputtering target. With the use of the target including polycrystallineIn-M-Zn oxide, an oxide semiconductor film having crystallinity can beeasily formed. Note that the atomic ratio of metal elements in the oxidesemiconductor film varies from the atomic ratio of those in theabove-described sputtering target, within a range of ±40% as an error.For example, when a sputtering target with an atomic ratio of In to Gaand Zn of 4:2:4.1 is used, the atomic ratio of In to Ga and Zn in theoxide semiconductor film may be 4:2:3 or in the vicinity of 4:2:3.

The energy gap of the oxide semiconductor film is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more. In thismanner, the amount of off-state current of a transistor can be reducedby using an oxide semiconductor having a wide energy gap.

The thickness of the oxide semiconductor film is greater than or equalto 3 nm and less than or equal to 200 nm, preferably greater than orequal to 3 nm and less than or equal to 100 nm, further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film. For example, the carrier density of the oxidesemiconductor film is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, further preferably lower than orequal to 1×10¹³/cm³, still further preferably lower than or equal to1×10¹¹/cm³. The carrier density of the oxide semiconductor film may bepreferably greater than or equal to 1×10⁵/cm³, further preferablygreater than or equal to 1×10⁷/cm³.

Note that without limitation to the materials given above, a materialwith an appropriate composition depending on intended semiconductorcharacteristics and electrical characteristics (e.g., field-effectmobility and threshold voltage) of a transistor can be used. In order toobtain intended semiconductor characteristics of the transistor, it ispreferable to set appropriate carrier density, impurity concentration,defect density, atomic ratio of a metal element to oxygen, interatomicdistance, density, and the like of the oxide semiconductor film.

Note that it is preferable to use, as the oxide semiconductor film, anoxide semiconductor film in which the impurity concentration is low anddensity of defect states is low, in which case the transistors can havemore excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (thenumber of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has few carrier generation sources, and thus can havea low carrier density. Thus, a transistor in which a channel region isformed in the oxide semiconductor film rarely has a negative thresholdvoltage (is rarely normally on). A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states and accordingly has few carrier traps insome cases. Further, the highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has an extremely lowoff-state current; even when an element has a channel width of 1×10⁶ μmand a channel length (L) of 10 μm, the off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drainvoltage) between a source electrode and a drain electrode of from 1 V to10 V.

Accordingly, the transistor in which the channel region is formed in thehighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film can have a small variation in electricalcharacteristics and high reliability. Charges trapped by the trap statesin the oxide semiconductor film take a long time to be released and maybehave like fixed charges. Thus, the transistor whose channel region isformed in the oxide semiconductor film having a high density of trapstates has unstable electrical characteristics in some cases. Asexamples of the impurities, hydrogen, nitrogen, alkali metal, alkalineearth metal, and the like are given.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and also causes oxygen vacancy in alattice from which oxygen is released (or a portion from which oxygen isreleased). Due to entry of hydrogen into the oxygen vacancy, an electronserving as a carrier is generated in some cases. Furthermore, in somecases, bonding of part of hydrogen to oxygen bonded to a metal elementcauses generation of an electron serving as a carrier. Thus, atransistor including an oxide semiconductor film which contains hydrogenis likely to be normally on. Accordingly, it is preferable that hydrogenbe reduced as much as possible in the oxide semiconductor film.Specifically, in the oxide semiconductor film, the concentration ofhydrogen which is measured by secondary mass spectrometry (SIMS) may belower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equalto 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹atoms/cm³, further preferably lower than or equal to 5×10¹⁸ atoms/cm³,further preferably lower than or equal to 1×10¹⁸ atoms/cm³, furtherpreferably lower than or equal to 5×10¹⁷ atoms/cm³, or furtherpreferably lower than or equal to 1×10¹⁶ atoms/cm³. Furthermore, in theoxide semiconductor film, the concentration of hydrogen which ismeasured by secondary mass spectrometry (SIMS) may be higher than orequal to 1×10¹⁶ atoms/cm³, preferably higher than or equal to 1×10¹⁷atoms/cm³.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor film, oxygen vacancy is increasedin the oxide semiconductor film, and the oxide semiconductor filmbecomes an n-type film. Thus, the concentration of silicon or carbon(the concentration is measured by SIMS) in the oxide semiconductor filmor the concentration of silicon or carbon (the concentration is measuredby SIMS) in the vicinity of an interface with the oxide semiconductorfilm is set to be lower than or equal to 2×10¹⁸ atoms/cm³. Theconcentration of silicon or carbon (the concentration is measured bySIMS) in the oxide semiconductor film or the concentration of silicon orcarbon (the concentration is measured by SIMS) at or near an interfacewith the oxide semiconductor film may be preferably set to be higherthan or equal to 1×10¹⁷ atoms/cm³, further preferably higher than orequal to 3×10¹⁷ atoms/cm³, still further preferably higher than or equalto 1×10¹⁸ atoms/cm³.

In addition, the concentration of alkali metal or alkaline earth metalof the oxide semiconductor film, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, or preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Thus, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film. In addition, the concentration ofalkali metal or alkaline earth metal of the oxide semiconductor film,which is measured by SIMS, may be higher than or equal to 5×10¹⁵atoms/cm³, or preferably higher than or equal to 1×10¹⁶ atoms/cm³.

Further, when nitrogen is contained in the oxide semiconductor film,electrons serving as carriers are generated and the carrier densityincreases, so that the oxide semiconductor film easily becomes n-type.Thus, a transistor including an oxide semiconductor film which containsnitrogen is likely to have normally-on characteristics. For this reason,nitrogen in the oxide semiconductor film is preferably reduced as muchas possible; the concentration of nitrogen which is measured by SIMS ispreferably set to be, for example, lower than or equal to 5×10¹⁸atoms/cm³. Furthermore, the concentration of nitrogen which is measuredby SIMS may be higher than or equal to 1×10¹⁶ atoms/cm³, preferablyhigher than or equal to 5×10¹⁶ atoms/cm³, further preferably higher thanor equal to 1×10¹⁷ atoms/cm³, still further preferably higher than orequal to 5×10¹⁷ atoms/cm³.

The oxide semiconductor film may have a non-single-crystal structure.The non-single crystal structure includes a c-axis aligned crystallineoxide semiconductor (CAAC-OS) which is described later, apolycrystalline structure, a microcrystalline structure, or an amorphousstructure, for example. Among the non-single crystal structure, theamorphous structure has the highest density of defect levels, whereasCAAC-OS has the lowest density of defect levels.

A structure of the oxide semiconductor film is described below.

An oxide semiconductor film is classified into a non-single-crystaloxide semiconductor film and a single crystal oxide semiconductor film.Alternatively, an oxide semiconductor is classified into, for example, acrystalline oxide semiconductor and an amorphous oxide semiconductor.

Examples of a non-single-crystal oxide semiconductor include a c-axisaligned crystalline oxide semiconductor (CAAC-OS), a polycrystallineoxide semiconductor, a microcrystalline oxide semiconductor, and anamorphous oxide semiconductor. In addition, examples of a crystallineoxide semiconductor include a single crystal oxide semiconductor, aCAAC-OS, a polycrystalline oxide semiconductor, and a microcrystallineoxide semiconductor.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image(also referred to as a high-resolution TEM image) of a bright-fieldimage and a diffraction pattern of the CAAC-OS film is observed.Consequently, a plurality of crystal parts are observed clearly.However, in the high-resolution TEM image, a boundary between crystalparts, that is, a grain boundary is not clearly observed. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a morphology reflecting unevenness of asurface where the CAAC-OS film is formed (hereinafter, a surface wherethe CAAC-OS film is formed is also referred to as a formation surface)or a top surface of the CAAC-OS film, and is arranged parallel to theformation surface or the top surface of the CAAC-OS film.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface, metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appears at around 31° and a peak of 2θ do not appear ataround 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancy in the oxide semiconductorfilm serves as a carrier trap or serves as a carrier generation sourcewhen hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the amount of oxygen vacancy is small) is referred to asa “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film is described.

A microcrystalline oxide semiconductor film has a region where a crystalpart is observed in a high resolution TEM image and a region where acrystal part is not clearly observed in a high resolution TEM image. Inmost cases, a crystal part in the microcrystalline oxide semiconductorfilm is greater than or equal to 1 nm and less than or equal to 100 nm,or greater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In a high resolution TEMimage of the nc-OS film, a grain boundary cannot be found clearly in thenc-OS film sometimes for example.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Note that there isno regularity of crystal orientation between different crystal parts inthe nc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Furthermore, a halo pattern is shown inan electron diffraction pattern (also referred to as a selected-areaelectron diffraction pattern) of the nc-OS film obtained by using anelectron beam having a probe diameter (e.g., larger than or equal to 50nm) larger than the diameter of a crystal part. Meanwhile, spots areshown in a nanobeam electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter close to, orsmaller than the diameter of a crystal part. Further, in a nanobeamelectron diffraction pattern of the nc-OS film, regions with highluminance in a circular (ring) pattern are shown in some cases. Also ina nanobeam electron diffraction pattern of the nc-OS film, a pluralityof spots is shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film.Therefore, the nc-OS film has a higher density of defect states than theCAAC-OS film.

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state as in quartz.

In the high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Further, a halo pattern is shown but a spot is not shown in a nanobeamelectron diffraction pattern of the amorphous oxide semiconductor film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may beseen. Furthermore, in the high-resolution TEM image, there are a regionwhere a crystal part is clearly observed and a region where a crystalpart is not observed. In the a-like OS film, crystallization by a slightamount of electron beam used for TEM observation occurs and growth ofthe crystal part is found sometimes. In contrast, crystallization by aslight amount of electron beam used for TEM observation is less observedin the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit cell of the InGaZnO₄ crystal has astructure in which nine layers of three In—O layers and six Ga—Zn—Olayers are layered in the c-axis direction. Accordingly, the spacingbetween these adjacent layers is equivalent to the lattice spacing onthe (009) plane (also referred to as d value). The value is calculatedto 0.29 nm from crystal structure analysis. Thus, focusing on latticefringes in the high-resolution TEM image, each of lattice fringes inwhich the lattice spacing therebetween is greater than or equal to 0.28nm and less than or equal to 0.30 nm corresponds to the a-b plane of theInGaZnO₄ crystal.

The density of an oxide semiconductor film might vary depending on itsstructure. For example, if the composition of an oxide semiconductorfilm is determined, the structure of the oxide semiconductor film can beestimated from a comparison between the density of the oxidesemiconductor film and the density of a single crystal oxidesemiconductor film having the same composition as the oxidesemiconductor film. For example, the density of the a-like OS film ishigher than or equal to 78.6% and lower than 92.3% of the density of thesingle crystal oxide semiconductor having the same composition. Forexample, the density of each of the nc-OS film and the CAAC-OS film ishigher than or equal to 92.3% and lower than 100% of the density of thesingle crystal oxide semiconductor having the same composition. Notethat it is difficult to deposit an oxide semiconductor film whosedensity is lower than 78% of the density of the single crystal oxidesemiconductor film.

Specific examples of the above description are given. For example, inthe case of an oxide semiconductor film with an atomic ratio ofIn:Ga:Zn=1:1:1, the density of single-crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Thus, for example, in thecase of the oxide semiconductor film with an atomic ratio ofIn:Ga:Zn=1:1:1, the density of an a-like OS film is higher than or equalto 5.0 g/cm³ and lower than 5.9 g/cm³. In addition, for example, in thecase of the oxide semiconductor film with an atomic ratio ofIn:Ga:Zn=1:1:1, the density of an nc-OS film or a CAAC-OS film is higherthan or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that single crystals with the same composition do not exist in somecases. In such a case, by combining single crystals with differentcompositions at a given proportion, it is possible to calculate densitythat corresponds to the density of a single crystal with a desiredcomposition. The density of the single crystal with a desiredcomposition may be calculated using weighted average with respect to thecombination ratio of the single crystals with different compositions.Note that it is preferable to combine as few kinds of single crystals aspossible for density calculation.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, an a-like OSfilm, a microcrystalline oxide semiconductor film, and a CAAC-OS film,for example.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “substantially parallel” indicates that the angleformed between two straight lines is greater than or equal to −30° andless than or equal to 30°. In addition, the term “perpendicular”indicates that the angle formed between two straight lines is greaterthan or equal to 80° and less than or equal to 100°, and accordinglyalso includes the case where the angle is greater than or equal to 85°and less than or equal to 95°. A term “substantially perpendicular”indicates that the angle formed between two straight lines is greaterthan or equal to 60° and less than or equal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

The semiconductor layer 402A has a channel formation region of thetransistor 101.

The semiconductor layer 402B has a channel formation region of thetransistor 102.

The semiconductor layer 402C has a channel formation region of thetransistor 103.

The semiconductor layer 402D has a channel formation region of thetransistor 104.

Note that the area of the semiconductor layer 402A is larger than eachof the areas of the semiconductor layers 402B, 402C, and 402D. The areaof the semiconductor layer 402B is larger than each of the areas of thesemiconductor layers 402C and 402D.

Note that the semiconductor layer 402A is provided on an inner side thanan edge portion of the conductive layer 401A. The semiconductor layer402B is provided on an inner side than an edge portion of the conductivelayer 401B. The semiconductor layer 402C is provided on an inner sidethan an edge portion of the conductive layer 401C. The semiconductorlayer 402D is provided on an inner side than an edge portion of theconductive layer 401D. Thus, steps of the semiconductor layers 402A to402D are not formed, which can suppress generation of defects.

The conductive layers 403A to 403I are in the same layer. Alternatively,the conductive layers 403A to 403I include the same material. Furtheralternatively, the conductive layers 403A to 403I are formed through astep of processing the same conductive film.

A material or a structure for the conductive layers 403A to 403I can beselected from materials or structures which can be used for theconductive layers 401A to 401D as appropriate.

The conductive layer 403A includes a region serving as one of a sourceelectrode or a drain electrode of the transistor 101. The conductivelayer 403A is connected to the semiconductor layer 402A. Alternatively,the conductive layer 403A includes a region in contact with thesemiconductor layer 402A. The conductive layer 403A is connected to theconductive layer 401D through a contact hole in the insulating layer404. Alternatively, the conductive layer 403A includes a region incontact with the conductive layer 401D. The conductive layer 403Aincludes a plurality of regions 403A1. Each of the plurality of regions403A1 is long along substantially the Y direction and overlaps with theconductive layer 401A with the semiconductor layer 402A interposedtherebetween. Furthermore, the conductive layer 403A includes a region403A2. The region 403A2 is long along substantially the X direction anddoes not overlap with the conductive layer 401A and the semiconductorlayer 402A.

The conductive layer 403B includes a region serving as the other of thesource electrode and the drain electrode of the transistor 101, a regionserving as a first electrode of the capacitor 105, and a region servingas the wiring 113. The conductive layer 403B is connected to thesemiconductor layer 402A. Alternatively, the conductive layer 403Bincludes a region in contact with the semiconductor layer 402A.Furthermore, the conductive layer 403B includes a plurality of regions403B1. Each of the plurality of regions 403B1 is long alongsubstantially the Y direction and overlaps with the conductive layer401A with the semiconductor layer 402A interposed therebetween.Furthermore, the conductive layer 403B includes a region 403B2. Theregion 403B2 is long along substantially the X direction and overlapswith the conductive layer 401A without the semiconductor layer 402Ainterposed therebetween. The conductive layer 403B includes an opening403B3 and an opening 403B4. Each of the openings 403B3 and 403B4 is longalong substantially the Y direction.

The conductive layer 403C includes a region serving as one of a sourceelectrode and a drain electrode of the transistor 102 and a regionserving as the wiring 113. The conductive layer 403C is connected to thesemiconductor layer 402B. Alternatively, the conductive layer 403Cincludes a region in contact with the semiconductor layer 402B.Furthermore, the conductive layer 403C includes a plurality of regions403C1. Each of the plurality of regions 403C1 is long alongsubstantially the Y direction and overlaps with the conductive layer401B with the semiconductor layer 402B interposed therebetween. Inaddition, the conductive layer 403C includes a region 403C2. The region403C2 is long along substantially the X direction and does not overlapwith the semiconductor layer 402B and the conductive layer 401B.

The conductive layer 403D includes a region serving as the other of thesource electrode and the drain electrode of the transistor 102 and aregion serving as a first electrode of the capacitor 106. The conductivelayer 403D is connected to the semiconductor layer 402B. Alternatively,the conductive layer 403D includes a region in contact with thesemiconductor layer 402B. The conductive layer 403D is connected to theconductive layer 401A through the contact hole in the insulating layer404. Alternatively, the conductive layer 403D includes a region incontact with the conductive layer 401A. Furthermore, the conductivelayer 403D includes a plurality of regions 403D1. Each of the pluralityof regions 403D1 is long along substantially the Y direction andoverlaps with the conductive layer 401B with the semiconductor layer402B interposed therebetween. Furthermore, the conductive layer 403Dincludes a region 403D2. The region 403D2 is long along substantiallythe X direction and overlaps with the conductive layer 401B without thesemiconductor layer 402B interposed therebetween. In addition, theconductive layer 403D includes an opening 403D3. The opening 403D3 islong along substantially the Y direction.

The conductive layer 403E includes a region serving as one of a sourceelectrode and a drain electrode of the transistor 103. The conductivelayer 403E is connected to the semiconductor layer 402C. Alternatively,the conductive layer 403E includes a region in contact with thesemiconductor layer 402C. Furthermore, the conductive layer 403 isconnected to the conductive layer 401C through the contact hole in theinsulating layer 404. Alternatively, the conductive layer 403E includesa region in contact with the conductive layer 401C.

The conductive layer 403F includes a region serving as the other of thesource electrode and the drain electrode of the transistor 103 and aregion serving as one of a source electrode and a drain electrode of thetransistor 104. The conductive layer 403F is connected to thesemiconductor layer 402C and the semiconductor layer 402D. In otherwords, the conductive layer 403F includes a region in contact with thesemiconductor layer 402C and a region in contact with the semiconductorlayer 402D. Furthermore, the conductive layer 403F is connected to theconductive layer 401B through the contact hole in the insulating layer404. In other words, the conductive layer 403F includes a region incontact with the conductive layer 401B.

The conductive layer 403G includes a region serving as the wiring 111.Furthermore, the conductive layer 403G is connected to the conductivelayer 401D through the contact hole in the insulating layer 404. Inother words, the conductive layer 403G includes a region in contact withthe conductive layer 401D.

The conductive layer 403H includes a region serving as the wiring 114.Furthermore, the conductive layer 403H is connected to the conductivelayer 401C through the contact hole in the insulating layer 404. Inother words, the conductive layer 403H includes a region in contact withthe conductive layer 401C.

The conductive layer 403I includes a region serving as the wiring 115and a region serving as the other of the source electrode and the drainelectrode of the transistor 104. Furthermore, the conductive layer 403Iis connected to the semiconductor layer 402D. In other words, theconductive layer 403I includes a region in contact with thesemiconductor layer 402D.

Note that the area of the opening 403B3 is larger than that of theopening 401A1, and the area of the opening 403B4 is larger than that ofthe opening 401A2. The opening 401A1 is provided on an inner side thanthe opening 403B3, and the opening 401A2 is provided on an inner sidethan the opening 403B4. Thus, a step of the conductive layer 403B due tothe conductive layer 401A is not formed, which can suppress generationof defects.

Note that the area of the opening 403D3 is larger than that of theopening 401B1. The opening 404A1 is provided on an inner side than theopening 403D3. Thus, a step of the conductive layer 403D due to theconductive layer 401B is not formed, which can suppress generation ofdefects.

Note that the region 403A2 of the conductive layer 403A does not overlapwith the conductive layer 401A and the semiconductor layer 402A, whereasthe region 403B2 of the conductive layer 403B overlaps with theconductive layer 401A without the semiconductor layer 402A interposedtherebetween. Note that the region 403B2 of the conductive layer 403Bmay overlap with the conductive layer 401A with the semiconductor layer402A interposed therebetween. The area where the conductive layers 403Aand 401A overlap with each other is smaller than the area where theconductive layers 403B and 401A overlap with each other. Thus, theparasitic capacitance between the conductive layers 403A and 401A can bereduced, and the parasitic capacitance between the conductive layers403B and 401A can be increased. Accordingly, an influence of thepotential of the wiring 111 on the gate of the transistor 101 can bereduced, and the capacitance of the capacitor 105 can be small, whichenables a reduction in a layout area.

Note that the region 403C2 of the conductive layer 403C does not overlapwith the conductive layer 401B and the semiconductor layer 402B, whereasthe region 403D2 of the conductive layer 403D overlaps with theconductive layer 401B without the semiconductor layer 402B interposedtherebetween. Note that the region 403D2 of the conductive layer 403Dmay overlap with the conductive layer 401B with the semiconductor layer402B interposed therebetween. The area where the conductive layers 403Cand 401B overlap with each other is smaller than the area where theconductive layers 403D and 401B overlap with each other. Thus, theparasitic capacitance between the conductive layers 403C and 401B can bereduced, and the parasitic capacitance between the conductive layers403D and 401B can be increased. Accordingly, an influence of thepotential of the wiring 113 on the gate of the transistor 102 can bereduced, and the capacitance of the capacitor 106 can be small, whichenables a reduction in a layout area.

There is no particular limitation on the property of a material and thelike of a substrate where the conductive layers 401A to 401D, thesemiconductor layers 402A to 402D, the conductive layers 403A to 403I,and the insulating layer 404 are formed as long as the material has heatresistance enough to withstand at least heat treatment to be performedlater. For example, a glass substrate, a ceramic substrate, a quartzsubstrate, a sapphire substrate, or the like may be used as thesubstrate. Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like may be used as thesubstrate. Furthermore, any of these substrates further provided with asemiconductor element may be used as the substrate. In the case where aglass substrate is used as the substrate, a glass substrate having anyof the following sizes can be used: the 6th generation (1500 mm×1850mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10thgeneration (2950 mm×3400 mm). Thus, a large-sized display device can bemanufactured.

Alternatively, a flexible substrate may be used as the substrate, andthe transistor may be provided directly on the flexible substrate.Further alternatively, a separation layer may be provided between thesubstrate and the transistor. The separation layer can be used when partor the whole of a semiconductor device formed over the separation layeris completed, separated from the substrate, and transferred to anothersubstrate. In such a case, the transistor can be transferred to asubstrate having low heat resistance or a flexible substrate as well.

Note that in this specification and the like, a transistor can be formedusing any of a variety of substrates, for example. The type of asubstrate is not limited to a certain type. As the substrate, asemiconductor substrate (e.g., a single crystal substrate or a siliconsubstrate), an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a metal substrate, a stainless steel substrate, asubstrate including stainless steel foil, a tungsten substrate, asubstrate including tungsten foil, a flexible substrate, an attachmentfilm, paper including a fibrous material, a base material film, or thelike can be used, for example. As an example of a glass substrate, abarium borosilicate glass substrate, an aluminoborosilicate glasssubstrate, a soda lime glass substrate, or the like can be given.Examples of the flexible substrate, the attachment film, the basematerial film, and the like are substrates of plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Anotherexample is a synthetic resin such as acrylic. Other examples arepolypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, or thelike. Other examples are polyester, polyamide, polyimide, aramid, epoxy,an inorganic vapor deposition film, paper, and the like. Specifically,the use of semiconductor substrates, single crystal substrates, SOIsubstrates, or the like enables the manufacture of small-sizedtransistors with a small variation in characteristics, size, shape, orthe like and with high current capability. A circuit using suchtransistors achieves lower power consumption of the circuit or higherintegration of the circuit.

Alternatively, a flexible substrate may be used as the substrate, andthe transistor may be provided directly on the flexible substrate.Further alternatively, a separation layer may be provided between thesubstrate and the transistor. The separation layer can be used when partor the whole of a semiconductor device formed over the separation layeris completed, separated from the substrate, and transferred to anothersubstrate. In such a case, the transistor can be transferred to asubstrate having low heat resistance or a flexible substrate as well.For the above separation layer, a stack including inorganic films, whichare a tungsten film and a silicon oxide film, or an organic resin filmof polyimide or the like formed over a substrate can be used, forexample.

In other words, a transistor may be formed using one substrate, and thentransferred to another substrate. Examples of a substrate to which atransistor is transferred include, in addition to the above substrateover which the transistor can be formed, a paper substrate, a cellophanesubstrate, an aramid film substrate, a polyimide film substrate, a stonesubstrate, a wood substrate, a cloth substrate (including a naturalfiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon,polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra,rayon, or regenerated polyester), and the like), a leather substrate,and a rubber substrate. When such a substrate is used, a transistor withexcellent properties or a transistor with low power consumption can beformed, a device with high durability, high heat resistance can beprovided, or reduction in weight or thickness can be achieved.

This embodiment can be combined with any of the other embodiments asappropriate. Thus, content (or may be part of the content) described inthis embodiment may be applied to, combined with, or replaced bydifferent content (or may be part of the different content) described inthe embodiment and/or content (or may be part of the content) describedin one or more different embodiments. Note that in each embodiment, acontent described in the embodiment is a content described withreference to a variety of diagrams or a content described with a textdescribed in this specification. In addition, by combining a diagram (orpart thereof) described in one embodiment with another part of thediagram, a different diagram (or part thereof) described in the sameembodiment, and/or a diagram (or part thereof) described in another orother embodiments, much more diagrams can be formed. This applies alsoto other embodiments.

Embodiment 5

In this embodiment, a display module and electronic appliances thatinclude a semiconductor device of one embodiment of the presentinvention are described with reference to FIG. 24 and FIGS. 25A to 25G.

In a display module 8000 illustrated in FIG. 24, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight 8007, a frame 8009, a printed board 8010, and a battery 8011are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device or the display device of one embodiment of thepresent invention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed so as to overlap with the display panel8006. A counter substrate (sealing substrate) of the display panel 8006can have a touch panel function. A photosensor may be provided in eachpixel of the display panel 8006 so that the touch panel 8004 canfunction as an optical touch panel.

The backlight 8007 includes a light source 8008. Note that although astructure in which the light sources 8008 are provided over thebacklight 8007 is illustrated in FIG. 24, one embodiment of the presentinvention is not limited to this structure. For example, a structure inwhich the light source 8008 is provided at an end portion of thebacklight 8007 and a light diffusion plate is further provided may beemployed. Note that the backlight 8007 need not be provided in the casewhere a self-luminous light-emitting element such as an organic ELelement is used or in the case where a reflective panel or the like isemployed.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 25A to 25G illustrate electronic appliances. These electronicappliances can include a housing 9000, a display portion 9001, a speaker9003, operation keys 9005 (including a power switch or an operationswitch), a connection terminal 9006, a sensor 9007 (a sensor having afunction of measuring or sensing force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared ray), amicrophone 9008, and the like.

The electronic appliances illustrated in FIGS. 25A to 25G can have avariety of functions. The electronic appliances illustrated in FIGS. 25Ato 25G can have a variety of functions, for example, a function ofdisplaying a variety of data (a still image, a moving image, a textimage, and the like) on the display portion, a touch panel function, afunction of displaying a calendar, date, time, and the like, a functionof controlling a process with a variety of software (programs), awireless communication function, a function of being connected to avariety of computer networks with a wireless communication function, afunction of transmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that functions that can be provided for theelectronic appliances illustrated in FIGS. 25A to 25G are not limited tothose described above, and the electronic appliances can have a varietyof functions. Although not illustrated in FIGS. 25A to 25G, theelectronic appliance may include a plurality of display portions.Furthermore, the electronic appliance may be provided with a camera andthe like and have a function of shooting a still image, a function ofshooting a moving image, a function of storing a shot image in a memorymedium (an external memory medium or a memory medium incorporated in thecamera), a function of displaying a shot image on the display portion,or the like.

The electronic appliances illustrated in FIGS. 25A to 25G are describedin detail below.

FIG. 25A is a perspective view illustrating a portable informationterminal 9100. A display portion 9001 of the portable informationterminal 9100 is flexible. Therefore, the display portion 9001 can beincorporated along a bent surface of a bent housing 9000. Furthermore,the display portion 9001 includes a touch sensor, and operation can beperformed by touching the screen with a finger, a stylus, or the like.For example, by touching an icon displayed on the display portion 9001,application can be started.

FIG. 25B is a perspective view illustrating a portable informationterminal 9101. The portable information terminal 9101 function as, forexample, one or more of a telephone set, a notebook, and an informationbrowsing system. Specifically, the portable information terminal 9101can be used as a smartphone. Note that although the speaker 9003, theconnection terminal 9006, the sensor 9007, and the like of the portableinformation terminal 9101 are not illustrated in FIG. 25B, they can beprovided in the same positions as the portable information terminal 9100in FIG. 25A. The portable information terminal 9101 can displaycharacters and image information on its plurality of surfaces. Forexample, three operation buttons 9050 (also referred to as operationicons or simply icons) can be displayed on one surface of the displayportion 9001. Furthermore, information 9051 indicated by dashedrectangles can be displayed on another surface of the display portion9001. Examples of the information 9051 include display indicatingreception of an incoming email, social networking service (SNS) message,and call; the title and sender of an email and SNS massage; the date;the time; remaining battery; and the reception strength of an antenna.Alternatively, the operation buttons 9050 or the like may be displayedin place of the information 9051.

FIG. 25C is a perspective view illustrating a portable informationterminal 9102. The portable information terminal 9102 has a function ofdisplaying information, for example, on three or more sides of thedisplay portion 9001. Here, information 9052, information 9053, andinformation 9054 are displayed on different sides. For example, a userof the portable information terminal 9102 can see the display (here, theinformation 9053) with the portable information terminal 9102 put in abreast pocket of his/her clothes. Specifically, a caller's phone number,name, or the like of an incoming call is displayed in a position thatcan be seen from above the portable information terminal 9102. Thus, theuser can see the display without taking out the portable informationterminal 9102 from the pocket and decide whether to answer the call.

FIG. 25D is a perspective view illustrating a wrist-watch-type portableinformation terminal 9200. The portable information terminal 9200 iscapable of executing a variety of applications such as mobile phonecalls, e-mailing, reading and editing texts, music reproduction,Internet communication, and a computer game. The display surface of thedisplay portion 9001 is bent, and images can be displayed on the bentdisplay surface. The portable information terminal 9200 can employ nearfield communication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible. Moreover, the portable information terminal 9200 includes theconnection terminal 9006, and data can be directly transmitted to andreceived from another information terminal via a connector. Chargingthrough the connection terminal 9006 is possible. Note that the chargingoperation may be performed by wireless power feeding without using theconnection terminal 9006.

FIGS. 25E, 25F, and 25G are perspective views illustrating a foldableportable information terminal 9201. FIG. 25E is a perspective viewillustrating the portable information terminal 9201 that is opened, FIG.25F is a perspective view illustrating the portable information terminal9201 that is being opened or being folded, and FIG. 25G is a perspectiveview illustrating the portable information terminal 9201 that is folded.The portable information terminal 9201 is highly portable when folded.When the portable information terminal 9201 is opened, a seamless largedisplay region is highly browsable. The display portion 9001 of theportable information terminal 9201 is supported by three housings 9000joined together by hinges 9055. By folding the portable informationterminal 9201 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9201 can be reversiblychanged in shape from an opened state to a folded state. For example,the portable information terminal 9201 can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm.

Electronic appliances described in this embodiment are characterized byhaving a display portion for displaying some sort of information. Notethat the semiconductor device of one embodiment of the present inventioncan also be used for an electronic appliance that does not have adisplay portion. The structure in which the display portion of theelectronic appliance described in this embodiment is flexible anddisplay can be performed on the bent display surface or the structure inwhich the display portion of the electronic appliance is foldable isdescribed as an example; however, the structure is not limited theretoand a structure in which the display portion of the electronic applianceis not flexible and display is performed on a plane portion may beemployed.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

This application is based on Japanese Patent Application serial no.2014-150532 filed with Japan Patent Office on Jul. 24, 2014, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a first transistor;a second transistor; and a capacitor, wherein one of a source and adrain of the first transistor is electrically connected to a firstwiring, wherein the other of the source and the drain of the firsttransistor is electrically connected to a second wiring, wherein theother of the source and the drain of the first transistor iselectrically connected to a first electrode of the capacitor, wherein agate of the first transistor is electrically connected to a secondelectrode of the capacitor, wherein the gate of the first transistor iselectrically connected to one of a source and a drain of the secondtransistor, wherein the other of the source and the drain of the secondtransistor is electrically connected to a third wiring, wherein thefirst transistor and the capacitor comprise a first conductive layer anda second conductive layer, wherein the first conductive layer comprisesa region serving as the source or the drain of the first transistor anda region serving as the first electrode of the capacitor, wherein thesecond conductive layer comprises a region serving as the gate of thefirst transistor and a region serving as the second electrode of thecapacitor, wherein the first conductive layer comprises a plurality offirst openings, wherein the second conductive layer comprises aplurality of second openings, and wherein the plurality of firstopenings and the plurality of second openings have a region overlappingeach other in a plane view.
 3. A semiconductor device comprising: afirst transistor; a second transistor; and a capacitor, wherein one of asource and a drain of the first transistor is electrically connected toa first wiring, wherein the other of the source and the drain of thefirst transistor is electrically connected to a second wiring, whereinthe other of the source and the drain of the first transistor iselectrically connected to a first electrode of the capacitor, wherein agate of the first transistor is electrically connected to a secondelectrode of the capacitor, wherein the gate of the first transistor iselectrically connected to one of a source and a drain of the secondtransistor, wherein the other of the source and the drain of the secondtransistor is electrically connected to a third wiring, wherein thefirst transistor and the capacitor comprise a first conductive layer anda second conductive layer, wherein the first conductive layer comprisesa region serving as the source or the drain of the first transistor anda region serving as the first electrode of the capacitor, wherein thesecond conductive layer comprises a region serving as the gate of thefirst transistor and a region serving as the second electrode of thecapacitor, wherein the first conductive layer comprises a first opening,wherein the second conductive layer comprises a second opening, andwherein an edge of the second opening is located outside the firstopening in a plane view.
 4. A semiconductor device comprising: a firsttransistor; a second transistor; and a capacitor, wherein one of asource and a drain of the first transistor is electrically connected toa first wiring, wherein the other of the source and the drain of thefirst transistor is electrically connected to a second wiring, whereinthe other of the source and the drain of the first transistor iselectrically connected to a first electrode of the capacitor, wherein agate of the first transistor is electrically connected to a secondelectrode of the capacitor, wherein the gate of the first transistor iselectrically connected to one of a source and a drain of the secondtransistor, wherein the other of the source and the drain of the secondtransistor is electrically connected to a third wiring, wherein a firstconductive layer of the first transistor comprises a region serving asthe first electrode of the capacitor, wherein a second conductive layerof the first transistor comprises a region serving as the secondelectrode of the capacitor, wherein the first conductive layer comprisesa first opening, wherein the second conductive layer comprises a secondopening, and wherein the first opening and the second opening partlyoverlap with each other in a plane view.
 5. The semiconductor deviceaccording to claim 2, further comprising a third transistor and a fourthtransistor.
 6. The semiconductor device according to claim 3, furthercomprising a third transistor and a fourth transistor.
 7. Thesemiconductor device according to claim 4, further comprising a thirdtransistor and a fourth transistor.
 8. The semiconductor deviceaccording to claim 5, wherein at least one of the first transistor, thesecond transistor, the third transistor, and the fourth transistorincludes an oxide semiconductor in a channel formation region.
 9. Thesemiconductor device according to claim 6, wherein at least one of thefirst transistor, the second transistor, the third transistor, and thefourth transistor includes an oxide semiconductor in a channel formationregion.
 10. The semiconductor device according to claim 7, wherein atleast one of the first transistor, the second transistor, the thirdtransistor, and the fourth transistor includes an oxide semiconductor ina channel formation region.
 11. A display module comprising: thesemiconductor device according to claim 2; and an FPC.
 12. A displaymodule comprising: the semiconductor device according to claim 3; and anFPC.
 13. A display module comprising: the semiconductor device accordingto claim 4; and an FPC.
 14. An electronic appliance comprising: thedisplay module according to claim 11; and an antenna, an operationbutton, or a speaker.
 15. An electronic appliance comprising: thedisplay module according to claim 12; and an antenna, an operationbutton, or a speaker.
 16. An electronic appliance comprising: thedisplay module according to claim 13; and an antenna, an operationbutton, or a speaker